AIR  SERVICE 
HANDBOOK 


Vol.  I 


AVIATION  SECTION 
SIGNAL  CORPS 


^^r.3^ 


J- 


AIR  SERVICE 
HANDBOOK 

VOL.  1 

AVIATION  SECTION,  SIGNAL  CORPS 


INTENTION  OF  BOOk 

The  contents  of  this  manual  have  been  coi.ied  from  various  works, 
and  the  chapters  which  apply  have  been  reproduced  word  for  word 

It  is  iutend.-d  thai  il  should  be  given  to  every  pilot,  and  that  a 
number  should  l)e  available  for  mechanics 

The  handbook  should  be  sufficient  for  elementary  training  in  rig- 
ping,  engines,  instruments,  magnetos,  meteorology  and  theory  of 
flight,  so  iSat  pupils  need  not  read  books  which  are  not  useful  or 
which  cove.-  the  same  ground,  in  other  words 


WASHINGTON  ^ 

GOVERNMENT  PRINTING  OFFICE 
1918 


4 


^War  "Depabtment, 
J)ocumeiit  No.  769. 


^ 

^ 


\i^  War  Department, 

Office  op  the  Chief  of  Staff, 

If  arch  26,  1918. 
The  following  Air  Service  handbook,  A  Course  of  Instruction  for 
Pilots  and  Mechanics,  prepared  in  the  Office  of  the  Chief  Signal 
J        Officer,  is  approved  and  published  for  the  information  and  guidance 
^        of  the  Schools  of  Military  Aeronautics. 

V2  (A.  G.  O.-NO.062.1.) 

^  By  order  of  the  Secretary  of  War. 

!^  D.  W.  KETCHAM, 

'^  ^  Colonel,  General  Staf. 

Official: 

"  H.  B.  McCAIN, 

\  The  Adjutant  General. 

i 

c 


483635 


CONTENTS. 


Page. 

I.  General  rules 7 

Routine  in  hangar 7 

General  organization  of  the  workshop 9 

II.  Rigging 13 

Rudiments  of  flight  and  stability 13 

General  rules 17 

Planes,  etc 19 

Struts,  wires,  cables 27-34 

Metal 35 

Wood 36 

Stresses  and  strains 37 

Handling  of  airplanes 39 

Keeping  an  airplane  in  good  condition 40 

III.  SailmaMng 43 

Tools  and  materials  used 43 

Patching 46 

Doping 47 

Markings  on  airplane 48 

IV.  Engine  material 49 

V.  The  gasoline  motor 54 

Detailed  description 58 

VI.  Engine  efficiency 69 

VII.  Propellers 74 

VIII.  Starting  the  engine 79 

IX.  Defects  in  the  engine 80 

X.  Ignition 85 

XI.  Motor  transport 91 

XII.  Instruments 95 

XIII.  Navigation  of  the  air 116 

XIV.  Notes  on  flying 124 

XV.  Meteorology 134 

XVI.  Theory  of  flight 146 

XVII.  Stability  of  machines 157 

XVIII.  Nomenclature 170 

Appendix 178 

5 


AIR  SERVICE  HANDBOOK. 


I.  GENERAL   RULES. 

THE    GENERAL   ROUTINE    IN    A    HANGAR. 

Because  the  endurance  and  air  worthiness  of  aircraft  largely 
depends  upon  the  care  which  is  spent  upon  them,  they  should  be 
well  looked  after  when  in  the  hangars.  Airplanes  should  not  be 
exposed  to  extremes  of  heat  and  cold.  However  well  seasoned 
wood  may  be,  if  it  is  allowed  to  absorb  moistui'e  it  will  invariably 
deteriorate.  Hangars  therefore  should  be  kept  dry  and,  as  far  as 
possible,  at  an  even  temperature. 

Cleanliness. — An  airplane  can  never  be  too  clean.  Rust,  mud, 
dirt,  and  superfluous  oil  should  be  at  once  removed  when  the 
machine  retui'ns  to  the  hangar. 

Supports. — Fui'ther,  an  airplane  once  housed  must  have  its  weight 
supported  in  such  a  manner  that  there  is  no  strain  on  the  shock 
absorbers.  The  tail  should  also  be  supported  so  that  there  is  not  a 
continual  strain  on  the  fuselage.  In  this  connection  it  must  be 
remembered  that  supports  should  be  so  placed  and  in  such  a  posi- 
tion that  the  main  weight  of  the  machine  is  directly  over  them. 
The  best  position  is  immediately  under  the  points  where  the  land- 
ing gear  struts  meet,  or  meet  the  longerons.  In  case  it  is  necessary 
to  support  the  wings  or  tail,  the  support  should  be  under  struts;  in 
the  case  of  the  wings,  under  the  interplane  struts  neai'est  to  the 
fuselage. 

Inspection. — Before  an  airplane  proceeds  on  a  flight  and  after  its 
return  all  parts — such  as  control  and  aileron  cables,  dope,  places 
where  cables  cross,  the  longerons,  etc. — must  be  thoroughly  exam- 
ined and  the  least  sign  of  wear  in  any  part  must  be  at  once  corrected. 
It  is  important  to  watch  the  weai'  of  control  wires  at  points  where 
they  pass  over  pulleys  or  fau-leads.  For  efficient  inspection  the 
machine  must  be  cleaned,  oil  and  grease  must  be  removed  before 
the  cables  can  be  properly  seen.  The  mere  fact  of  cleaning  a  machine 
insures  that  every  part  has  at  least  been  looked  at. 

All  engines  must  be  thoroughly  tested  before  flying  and  after  any 
repair's  or  overhauling  has  been  effected.  Once  a  week,  at  least,  a 
thorough  routine  examination  must  be  made  of  all  struts,  internal 

7 


8  AIR  SERVICE  HANDBOOK. 

bracing  wires  of  fuselage,  etc.,  with  a  view  to  checking  any  damage 
or  want  of  alignment.  If  an  airplane  makes  a  forced  descent  and 
has  to  be  left  in  the  open  it  is  important  to  guard  against  possible 
contingencies,  and  it  should  be  looked  after  as  described  under 
"Cross  country  flying." 

The  planes  of  a  machine  must  be  cleaned  dii'ectly  after  a  flight, 
to  prevent  the  oil  and  dirt  soaking  into  the  machine.  Oil  deterior- 
ates wood  and  the  machine  may  therefore  lose  its  factor  of  safety. 

At  the  end  of  a  hundred  hoiu-s'  flying,  the  machine  should  be 
inspected  by  the  engineer  officer,  who  may  or  may  not  allow  it  to 
be  flown  farther. 

Any  repairs  necessary  to  a  machine  should  be  done  at  once,  as  a 
machine  is  often  wanted  in  a  hurry 

Tires  of  machines  are  injured  by  oil  and  grease.  No  oil  there- 
fore should  be  allowed  to  collect  on  the  floor  of  the  hangar. 

Hangar  floors.  —Floors  of  hangars  may  be  kept  clean  by  the  appli- 
cation of  hot  water  and  caustic  soda.  In  this  connection,  it  should 
be  remembered  that  caustic  soda  rots  leather  boots  so  that  wooden 
clogs  should  be  used  by  the  men  engaged  in  this  work.  Sawdust 
must  not  be  allowed  as  it  accumulates  dirt.  Sawdust  and  old  rags 
which  have  become  oily  are  liable  to  spontaneous  combustion  if 
they  are  left  in  the  sun  or  near  the  sides  of  a  corrugated  iron  build- 
ing. Sawdust  is  only  permissible  in  a  tray  in  order  to  catch  the 
waste  oil  from  the  engine. 

Engine  stands. — Stands  should  be  provided  for  engines  to  rest  on 
when  they  are  taken  out  of  airplanes. 

Clothing. — Pegs  should  be  provided  for  aviation  clothing  and  hel- 
mets and  also  for  the  mechanician's  coats,  which  are  changed  for 
overalls  when  the  men  come  to  work.  No  clothing  should  be  allowed 
to  lie  about. 

Fire. — Owing  to  the  inflammable  nature  of  an  aii-plane  building 
and  the  large  value  of  the  articles  kept  in  it,  every  precaution  must 
be  taken  against  fire.  Fire  drill  should  be  held  periodically.  Buck- 
ets of  sand  and  water  must  be  kept  completely  filled  in  convenient 
places  in  each  hangar.  At  least  one  fire  extinguisher,  such  as 
"Pyrene,"  should  be  kept  in  each  hangar  ready  for  use,  and  all 
men  should  know  how  to  use  this  apparatus. 

Notice  board. — A  bulletin  board  should  be  provided  in  the  hangar 
of  each  section  on  which  orders,  etc.,  should  be  posted. 

Spares. — Only  the  authorized  spares  may  be  kept  in  the  hangars. 
The  tendency  to  accumulate  unauthorized  parts  must  be  checked. 
Care  must  be  taken  that  spare  parts,  where  applicable,  are  kept 
properly  oiled  or  greased  to  prevent  them  rusting.  Every  part 
must  bear  a  label,  showing  exactly  what  it  is  and  to  what  it  belongs. 


AIR   SERVICE   HANDBOOK.  9 

No  mechanician  should  be  allowed  the  opportunity  of  making  a 
private  collection  of  spare  parts  for  use  in  effecting  repaii-s.  Pigeon 
holes  should  be  provided  for  engine  parts  and  various  small  stores, 
which  should  be  labeled.  When  drawing  new  parts  from  the  supply- 
room  the  mechanician  should,  as  a  general  rule,  and  if  possible, 
hand  in  at  the  same  time  the  corresponding  broken  parts.  Con- 
demned parts  should  be  clearly  marked  and  kept  in  a  special  ])lace, 
so  that  there  is  no  possibility  of  their  being  used  again  by  a  careless 
workman.  Spare  planes  should  be  stored  in  such  a  manner  that 
their  weight  is  evenly  supported.  One  plane  must  not  be  allowed 
to  butt  into  another.  It  has  been  found  best  to  suspend  planes  by 
means  of  canvas  slings  hung  from  overhead.  Within  the  loop  of 
the  slings  there  must  be  a  wooden  batten  about  2^  inches  wide,  so 
that  the  leading  edge  of  the  plane  is  supported  the  whole  way  along. 

Records. — Rough  records  should  be  kept  in  each  section  in  which 
all  details  of  flights,  overhauls,  expenditm'e,  fuel,  and  oil,  etc.,  are 
entered  whenever  each  event  occurs  and  are  of  assistance  in  making 
the  record  an  accurate  history  of  the  aii'plane  and  engine.  Log 
books  must  in\ariably  be  made  up  to  date,  signed  and  forwarded  at 
the  same  time  as  an  engine  or  airplane  is  transferred  from  one  unit 
to  another. 

Smoking. — Smoking  in  the  hangar  is  strictly  prohibited. 

GENERAL  ORC.VNIZATION  OP  THE  WORKSHOP. 

Personnel. — The  personnel  available  should  be  divided  into  three 
separate  departments,  with  a  master  signal  electrician  in  charge  of 
each.  A  commissioned  officer  should  exercise  general  supervision 
over  each  department. 

The  three  departments  are: 
(a)  Aero  repair  shop. 
(6)  Dope  shop, 
(c)  Machine  shop. 

It  is  desirable  that  the  aii]jlane  and  dope  work  be  conducted  in 
a  separate  building  from  the  engine  and  machine  work. 

Care. — ^All  mechanicians  must  be  made  to  realize  that  the  greatest 
care  and  attention  to  the  minutest  details  is  absolutely  necessary. 

Bench. — Where  airplane  hangars  or  workshops  are  provided  with 
benches  and  \dses  it  is  convenient  that  the  benches  be  fitted  with 
lock-up  drawers  for  the  storage  of  tools. 

Benches  must  not  be  allowed  to  become  mere  shelves  for  the 
assortment  of  rubbish,  spare  parts,  and  discarded  breakages.  No 
article  should  be  kept  on  a  bench  close  to  where  some  particular 
Work  is  on  hand  which  has  not  a  direct  bearing  on  that  work.  Boxes 
should  be  provided  in  which  metal  such  as  brass  or  steel  fittings 


10  AIR  SERVICE   HANDBOOK. 

may  be  kept.  At  regular  intervals,  not  less  than  once  each  week, 
all  rubbish  must  be  removed. 

Tool  boxes. — Places  should  be  allotted  for  the  mechanics'  tool 
boxes,  and  their  contents  should  be  periodically  inspected.  A  list 
of  the  correct  contents  of  the  box  should  be  pasted  inside  the  lid. 

Care  of  machinery.- — Only  the  mechanics  authorized  by  the  engineer 
officer  should  be  allowed  to  use  the  lathes,  saws,  etc.,  with  which 
the  shops  are  provided  and  to  start  the  motors  for  dri\dng  these 
machines. 

With  electrically  driven  machinery  care  must  be  taken  that  all 
switches  are  "off"  before  the  shops  are  closed  at  the  end  of  the  day. 
Lathes,  etc.,  and  their  accessory  parts  must  be  kept  properly  oiled 
and  greased  and  free  from  rust.  All  belting  must  be  provided  with 
suitable  guards.  All  flat-faced  surfaces  in  lathes  should  be  suitably 
protected  by  wood  to  prevent  them  from  being  damaged  by  tools 
falling  on  them.  All  shavings,  sawdust,  and  metal  turnings  must 
be  cleared  away  from  the  machines  daily.  The  metal  turnings 
should  be  preserved  for  future  use  or  sale,  different  metals  being 
kept  separate. 

Examination  and  dismantling  of  airplanes  .—This  work,  if  it  is  made 
a  matter  of  routine,  is  simple  and  occupies  a  small  amount  of  time. 
The  following  is  the  system  which  has  been  found  most  suitable: 

(a)  In  the  case  of  serious  damage  or  periodical  overhaul  the 
airplane  must  be  taken  at  once  to  the  shops  by  the  men  of  the  section 
to  which  it  belongs. 

(6)  The  engineer  officer  then  carries  out  his  detailed  examination 
and  prepares  a  statement  setting  out  in  detail  the  general  condition 
of  the  machine. 

(c)  The  airplane  is  then  stripped  and  all  parts  labeled. 

(d)  In  all  cases  in  which  the  machine  has  been  in  an  accident 
the  engine  must  be  removed  for  a  thorough  examination  and  over- 
haul. For  this  purpose  it  must  be  turned  over  to  the  machine 
shop. 

(e)  The  parts  which  are  not  repairable  are  removed  to  the  author- 
ized place  and  all  small  stores,  such  as  turnbuckles,  bolts,  etc.,  which 
are  apparently '  still  serviceable  are  handed  into  the  shop  stock 
room.  In  the  stock  room  they  are  kept  separate  from  the  other 
spares  until  they  have  been  pronounced  ser\dceable  or  otherwise 
by  the  officer  in  charge. 

('/)  The  parts  which  can  he  repaired  and  made  fit  for  service  are 
labeled  and  sent  to  the  department  concerned  where  they  will 
await  their  turn  for  repair. 

(g)  The  undamaged  parts  are  handed  into  the  stock  room  properly 
labeled.    These  should  be  taken  on  temporary  charge  as  spares  in 


AIR  SERVICE  HANDBOOK.  11 

the  workshop  store,  until  the  airplane  is  again  ready  for  them.  If 
the  airplane  can  not  be  repaired  the  undamaged  parts  must  be 
taken  on  permanent  charge  in  the  store  account  of  the  unit. 

(h)  All  instruments  requii'ing  repairs  should  l/e  returned  to  the 
stockroom  and  the  engineer  oflicer  should  decide  whether  the  instru- 
ments are  to  be  returned  to  the  makers  for  repair  or  not. 

(i)  All  parts  repairable  or  sound  should  be  thoroughly  cleaned 
before  being  turned  over  to  the  supply  officer. 

(j)  Parts  of  airplanes  while  awaiting  erection  must  be  properly 
supported  or  stored.  When  the  airplane  is  being  erected  all  parts 
intended  for  that  particular  machine  must  be  kept  together  so  as  to 
avoid  using  wrong  ])arts. 

Engine  repair  wrk.- — ^A  system  must  be  established  and  worked 
on  whenever  an  engine  is  taken  down  for  repair  and  reassembled. 

Suitable  stands  must  be  provided  on  which  to  place  engines. 

Trays  divided  up  into  compartments  in  which  to  put  each  part  of 
the  engine  as  it  is  removed  are  a  necessity.  It  is  a  good  thing  to 
have  each  compartment  numbered  and  the  parts  of  each  cylinder 
and  its  attachments  put  into  the  compartment  coi'responding  to  its 
number  in  the  engine.  A  little  shelf  can  conveniently  be  con- 
structed on  the  outside  of  these  compartments  on  which  to  lay  the 
tools  so  as  to  prevent  them  getting  lost  or  dirty. 

Only  in  cases  of  urgency  should  parts  of  one  engine  be  used  to 
complete  another.  If  such  a  course  is  necessary  the  parts  so  used 
should  be  numbered  afresh  so  as  to  correspond  with  the  numbering 
of  the  engine  in  which  they  are  used.  Thus,  if  "number  5"  piston 
of  one  engine  is  to  be  used  as  '"number  7"  in  another,  it  should  be 
renumbered  'number  7." 

When  reasseml)ling  an  engine  the  utmost  care  is  necessary  to 
insure  that  all  parts  are  free  from  grit  and  dirt.  Gasoline  baths  are 
a  necessity. 

Every  part  should  be  thoroughly  oiled  before  l^eing  replaced. 

Any  metal  part  which  has  been  bent  should  on  no  account  be 
straightened  and  replaced  in  the  engine  without  the  knowledge  of 
the  engineer  officer.  As  a  general  rule  such  bent  parts  must  not  be 
straightened  and  used  again. 

When  the  engine  has  been  overhauled  and  tested  it  should  bear  a 
label  showing  date  of  test,  time  run,  and  number  of  revolutions 
obtained.  It  should  then  be  put  to  one  side  and  greased  pending 
a  necessity  arising  for  its  use  in  an  ahplane. 

Engines  must  be  turned  daily  by  hand. 

Logbooks  must  be  kept,  made  up  to  date,  showing  work  done  on 
the  engine  and  these  books  must  always  accompany  the  engines. 


12  AIR   SERVICE   HANDBOOK. 

The  care  of  tools. — Protect  all  edges.  Keep  all  edged  tools  sharp. 
If  you  dull  a  tool  by  using  it  sharpen  before  returning  it  to  its  place. 

In  sharpening  an  edged  tool  do  not  blunt  it.  Grind  an  angle  and 
keep  it  until  the  tool  becomes  sharp.  If  you  round  off  an  edged  tool 
you  ruin  it. 

Do  not  use  a  file  like  a  hack  saw,  as  most  files  are  made  to  cut  on 
the  forward  stroke  only.  Keep  files  in  a  case  where  they  do  not  rub 
against  each  other  and  keep  them  free  fi'om  oil  and  gi'ease.  When 
using  a  file  always  put  a  handle  on  it.  Have  a  file  cleaner  handy 
and  use  it  often. 

For  a  fine  finish  on  steel  do  not  use  a  file,  use  an  oilstone. 

Do  not  keep  soldering  acid  near  tools  nor  handle  tools  after  using 
acid  without  fu'st  washing  your  hands. 

Have  your  list  of  tools  pasted  in  the  top  of  your  chest  and  check 
your  tools  each  evening  before  quitting. 

Keep  bits  sharp  and  in  a  case  and  do  not  expect  to  drill  a  true  hole 
■with  a  dull  bit. 

Do  not  use  a  steel  hammer  on  metal  parts,  use  a  brass  or  rawhide 
hammer,  or  copper  or  brass  drift. 

Oil  your  tools  often  in  rainy  or  wet  weather. 

Do  not  keep  sulphur  or  salt  near  your  tools. 

Always  oil  the  steel  tape  before  putting  it  away. 

Do  not  use  a  monkey  wrench  as  a  hammer. 

Do  not  use  a  screw  driver  as  a  punch,  drift,  or  lever,  and  keep  the 
sides  of  the  point  parallel. 

Do  not  tise  pliers  on  nuts.  Do  not  use  a  24-inch  monkey  wrench 
on  a  quarter-inch  nut.  Do  not  use  an  end  wrench  on  a  nut  unless 
it  fits  proi^erly.  The  same  remark  applies  to  all  wrenches  on  nuts, 
and  to  screw  drivers  on  screws. 

Keep  yoiu'  fine  measiu*ing  tools  in  a  case. 

Do  not  hold  work  that  is  being  heated  with  a  torch  with  a  pair  of 
pliers;  use  regular  tongs. 

Use  only  regular  nippers  for  cutting  piano  wire.  The  ordinary 
side-cutting  pliers  are  usually  not  strong  enough  for  this. 

In  using  a  tap  always  be  sure  that  the  right  size  hole  has  been 
drilled.  In  using  taps  and  dies  use  plenty  of  lard  oil  on  the  work 
and  be  careful  of  backing  up  the  tap  or  die.  Always  read  instructions 
furnished  by  the  manufacturer.  In  cutting  large,  heavy  stock  it  is 
better  to  take  more  than  one  cut.  Always  clean  taps  and  dies  before 
putting  them  away. 

Do  not  use  a  Stilson  wrench  on  nuts. 

Sharpen  your  tools  over  all  the  surface  of  an  oilstone  and  not  only 
in  one  place. 

Always  keep  Ihe  cutting  edge  of  your  saw  well  protected. 


AIR   SERVICE   HANDBOOK. 


1» 


It  is  useful  to  have  a  place  for  each  tool  so  that  they  can  not  shift 
when  the  boxes  are  put  on  the  motor  transport  in  a  hurry. 

Each  mechanic  is  advised  to  keep  a  memorandum  book  in  his 
possession  in  which  he  may  record  such  notes  as  may  be  considered 
useful  to  him. 

II.     RIGGING. 


RUDIMENTS  OF  FLIGHT  AND  STABILITY. 

In  order  that  a  pilot  may  take  an  interest  in  the  rigging  of  his 
machine  he  should  have  a  sound  idea  of  the  rudiments  of  flight  and 
stability.  He  need  not  know  how  to  design  a  machine,  as  this  is  a 
highly  technical  procedure  which  is  done  by  the  machine  designers. 
The  pilot  should  know  how  to  fly  any  machine  he  is  given  to  the  best 


advantage 


-rl. 


J'-.'tcV 


4  U>  UJ^WC 


Fig.  1, 

Flight  is  secured  by  driving  through  the  air  a  surface  or  surfaces 
inclined  to  the  direction  of  motion.  The  surface  meeting  the  air 
tends  to  drive  the  air  downward  and  this  causes  a  resultant  reaction 
on  the  surface.  The  resultant  acts  approximately  at  right  angles  to 
the  plane  or,  if  the  plane  is  curved,  to  the  chord. 

The  total  reaction  on  the  aerofoil  can  be  considered  as  made  up  of 
two  forces,  one  acting  in  a  vertical  direction  and  one  in  a  horizontal. 

Lift. — The  lifting  planes  by  being  driven  through  the  air  secure 
a  lift,  and  when  the  speed  is  great  enough  the  lift  will  become  greater 
than  the  weight  of  the  airplane,  w^hich  must  then  rise.  Thus,  the 
vertical  part  of  the  total  reaction  on  the  plane  is  called  "Lift." 
Bear  in  mind  that  the  lift  is  always  trying  to  collapse  the  planes 
upward . 

Drift. — The  resistance  of  the  air  to  the  passage  of  an  airplane  is 
known  as  "Drift."    Thus,  the  horizontal  part  of  the  total  reaction 


14  AIR  SERVICE  HANDBOOK. 

on  the  plane  is  called  "Drift."  This  is  overcome  by  the  "thrust" 
of  the  propeller,  which  thrusts  the  airplane  (or  drags  it)  through  the 
air  and  so  overcomes  the  drift.  Bear  in  mind  that  the  drift  is  always 
trying  to  collapse  the  planes  backward. 

You  will  see  from  the  above  diagram  that  there  are  four  forces  to 
consider.  The  lift,  which  is  opposed  to  the  weight,  and  the  thrust, 
which  is  opposed  to  the  drift.  The  lift  is  useful;  the  drift  is  the  re- 
verse of  useful.  The  proportion  of  lift  to  drift  is  known  as  the  "  Lift- 
drift  ratio."  This  is  of  paramount  importance,  for  upon  it  depends 
the  efficiency  of  the  airplane.  In  rigging  an  airplane  the  greatest 
care  must  be  taken  to  preserve  the  lift-drift  ratio.  Always  keep  that 
in  mind.  This  means  that  the  lifting  planes  of  a  machine  must  not 
be  damaged,  that  the  adjustments  of  the  machine  are  exactly  in 
accordance  with  the  rigging  diagram,  and  that  all  work  is  done 
neatly  and  carefully. 

Angle  of  incidence. — The  angle  of  incidence  is  the  inclination  of 
the  lifting  surfaces.  If  the  angle  of  incidence  is  increased  over  the 
angle  specified  in  your  rigging  instructions,  then  both  the  lift  and 
drift  are  increased  also,  and  the  drift  is  increased  in  greater  pro- 
portion than  the  lift.  If,  however,  the  angle  of  incidence  is  de- 
creased, then  the  lift  and  drift  are  decreased  and  the  lift  decreases 
in  greater  proportion  than  does  the  drift.  You  see  then  that  in  each 
case  the  efficiency  is  spoiled,  because  the  proportion  of  lift  to  drift 
is  not  so  good  as  would  otherwise  be  the  case. 

Balance. — The  whole  weight  of  the  airplane  is  balanced  upon,  or 
slightly  forward  of,  the  center  of  the  lift. 

If  the  weight  is  too  far  forward,  then  the  machine  is  nose  heavy. 

If  the  weight  is  too  far  behind  the  center  of  the  lift  then  the  air- 
plane is  tail  heavy. 

In  either  case  an  adjustment  must  be  made  which  spoils  the 
efficiency  of  the  machine. 

Stability. — By  stability  of  the  airplane  is  meant  the  tendency  of 
the  airplane  to  remain  upon  an  even  keel  and  to  keep  its  course; 
that  is  to  say,  not  to  fly  one  wing  down,  tail  down,  or  nose  down,  or 
to  try  and  txurn  off  its  course. 

Directional  stability. — By  directional  stability  is  meant  the  natural 
tendency  of  the  airplane  to  remain  upon  its  course.  If  this  did  not 
exist,  it  would  be  continually  trying  to  turn  to  right  or  to  left,  and 
the  pilot  would  not  be  able  to  control  it. 

For  the  airplane  to  have  directional  stability  it  is  necessary  for  it 
to  have,  in  effect,  more  keel  surface  behind  its  turning  axis  than 
there  is  in  front  of  it. 

By  keel  surface  is  meant  everything  you  can  see  when  you  look 
at  the  airplane  from  the  side  of  it — the  sides  of  the  body,  landing 


AIR  SERVICE   HANDBOOK.  16 

gear,  wires,  struts,  etc.  Directional  stability  is  sometimes  known  as 
"weather-cock  stability." 

If  in  the  case  of  the  "weather  cock"  there  was  too  much  keel 
surface  in  front  of  its  turning  axis,  which  is  the  point  upon  which  it 
is  pivoted,  it  would  turn  around  the  wrong  way;  and  this  is  just  what 
would  happen  in  the  case  of  the  airplane. 

Directional  stability  will  be  badly  affected  if  there  is  more  drift 
(i.  e.,  resistance)  on  one  side  of  the  airplane  than  there  is  on  the  other. 
This  may  be  caused  by  the  following: 

1.  The  angle  of  incidence  of  the  main  planes  or  the  tail  plane 
may  be  Avrong.  If  the  angle  of  incidence  on  one  side  of  the  machine 
is  not  what  it  should  be,  that  will  cause  a  difference  in  the  drift 
between  the  two  sides  of  the  aii^plane,  with  the  result  that  it  will 
turn  off  its  course. 

2.  If  the  alignment  of  the  fuselage  or  fin  in  front  of  the  rudder 
or  the  stream-line  struts  is  not  absolutely  correct,  that  is  to  say, 
if  they  are  turned  a  little  to  the  right  or  left  instead  of  being  in  line 
with  the  center  of  the  machine  in  the  case  of  the  fin  and  dead  on  in 
the  direction  of  flight,  they  will  act  as  an  enormous  rudder  and  cause 
the  machine  to  turn  off  its  covu-se. 

3.  If  the  dihedral  angle  is  wrong  that  may  have  a  bad  effect.  It 
may  result  in  the  propeller  not  thrusting  from  the  center  of  the 
drift,  in  which  case  it  will  pull  the  machine  a  little  .=iideways  and  out 
of  its  course. 

4.  If  the  struts  and  stream  lines  on  the  wires  are  not  adjusted  to  he 
dead  on  in  the  line  of  flight,  then  they  will  produce  additional  drift 
on  their  side  of  the  airplane,  with  the  result  that  it  will  turn  off  its 
course. 

5.  Distorted  surfaces  may  cause  the  airplane  to  be  dii'ectionally 
bad.  The  planes  are  "cambered  ";  that  is,  curved  to  go  through  the 
air  with  the  least  possible  drift.  If  perhaps  owing  to  the  leading 
edge  spars  or  trailing  edge  getting  bent,  the  curvature  is  spoiled, 
with  the  result  that  the  amount  of  drift  on  one  side  of  the  airplane 
is  altered,  causing  the  macliiue  to  have  a  tendency  to  turn  off  its 
course. 

Lateral  stability. — By  lateral  stability  is  meant  the  sideways  bal- 
ance of  the  machine.  The  only  possible  thing  that  could  make  the 
machine  fly  one  wing  down  is  that  there  is  more  lift  on  one  side  than 
there  is  on  the  other.     This  may  be  due  to — 

1.  The  angle  of  incidence  may  be  wrong.  If  the  angle  of  inci- 
dence is  too  great,  it  will  produce  more  lift  on  that  aide  than  on  the 
other.  The  result  will  be  that  the  machine  flies  one  wing  down. 
This  remark  also  applies  to  too  little  incidence  on  one  wing. 


16  AIR  SERVICE   HANDBOOK. 

2.  Distorted  surfaces:  If  the  planes  are  distorted,  then  their  cam- 
ber is  spoiled  and  the  lift  will  not  be  the  same  on  both  sides  of  the 
airplane. 

3.  If  stability  is  not  horizontal,  it  will  cause  a  twisting  movement. 

Longitudinal  stability. — By  longitudinal  stability  is  meant  the  fore- 
and-aft  balance.  If  this  is  not  correct,  the  machine  will  try  to  fly 
nose  or  tail  down.     This  may  be  due  to — 

1.  The  stagger  may  be  wrong.  The  top  plane  may  have  drifted 
back  a  little,  and  this  may  be  due  to  some  of  the  wires  having  elon- 
gated their  loops  or  having  pulled  the  fittings  into  the  wood.  If 
the  top  plane  is  not  staggered  forward  to  the  correct  degree,  it  means 
that  the  whole  of  the  lift  of  the  airplane  is  moved  backward,  so  that 
it  will  have  a  tendency  to  lift  the  tail;  that  is,  it  will  become  nose 
heavy.  A  quarter  inch  error  in  the  stagger  makes  a  considerable 
difference. 

2.  If  the  angle  of  incidence  of  the  main  planes  is  too  great,  it  will 
produce  an  excess  of  lift,  which  will  tend  to  lift  the  nose  of  the  ma- 
chine.    If  the  angle  is  too  small,  the  opposite  happens. 

3.  When  the  machine  is  longitudinally  out  of  balance  the  usual 
thing  is  for  the  rigger  to  rush  to  the  tail  plane,  thinking  that  its  ad- 
justment relative  to  the  fuselage  must  be  wrong.  This  is  the  least 
likely  reason.  It  is  much  more  likely  to  be  one  of  the  first  two,  or, 
more  probable  still,  that  the  fuselage  has  warped  upward.  This  gives 
the  tail  plane  an  incorrect  angle  of  incidence.  This  is  due  to  bad 
landings  or  to  allowing  the  machine  to  rest  in  the  hangar  with  its 
its  tail  on  the  ground,  so  that  it  always  has  a  certain  amount  of 
weight  on  it  and  it  gets  no  rest. 

4.  If  the  above  three  points  are  correct,  there  is  a  possibility  that 
the  tail  plane  itself  has  assumed  a  wrong  angle  of  incidence.  In 
such  event,  if  the  machine  is  nose  heavy,  the  tail  plane  should  be 
given  a  smaller  angle  of  incidence.  If  the  machine  is  tail  heavy, 
then  the  tail  plane  must  be  given  a  large  angle  of  incidence,  but 
be  careful  not  to  give  the  tail  plane  too  great  an  angle  of  incidence. 
The  longitudinal  stability  of  the  airplane  entirely  depends  on  the 
tail  plane  being  at  a  much  smaller  angle  of  incidence  than  the  main 
plane,  and  if  you  cut  the  difference  down  too  much  the  machine  will 
become  uncontrollable.  Sometimes  the  tail  plane  is  set  on  the  ma- 
chine at  the  same  angle  of  incidence  as  the  main  planes,  but  it  ac- 
tually engages  the  air  at  a  lesser  angle,  owing  to  the  air  being  de- 
flected downward  by  the  main  planes. 

Propeller  torque. — Owing  to  propeller  torque,  the  airplane  has  a 
tendency  to  turn  over  sideways  in  the  direction  opposite  to  that  in 
which  the  propeller  revolves.  In  some  machines  this  tendency  is 
rather  marked,  and  this  is  offset  by  increasing  the  angle  of  incidence 
on  the  side  tending  to  fall  and  by  decreasing  the  angle  of  incidence 


AIR  SERVICE  HANDBOOK.  17 

the  same  amount  ou  the  side  tending  to  rise.  In  this  way  uioii- 
lift  is  secured  ou  one  side  of  the  machine  than  on  the  olher,  so  that 
the  tendency  to  overturn  is  corrected. 

Wash  in. — When  the  angle  of  incidence  toward  the  tij)  of  the  main 
plane  is  increased  the  plane  is  said  to  have  wash  in. 

Wash  out.— When  the  angle  of  incidence  i.*  decreased  it  is  called 
wash  out. 

Sometimes  wash  out  is  given  to  both  sides  of  a  main  plane.  This 
decreases  the  drift  toward  the  wing  tips,  and  consequently  decreases 
the  effect  of  gusts  upim  them.  It  also  renders  the  ailerons  more 
effective. 

Importance  oj ijood  riyyiiKj.—lt  is  imjiossible  to  exaggerate  the  im- 
portance of  care  and  accuracy  in  rigging.  The  lives  of  the  crew, 
the  speed  and  climb  of  the  airplane,  its  control  and  general  elti- 
ciency  in  flight,  and  its  duration  as  a  useful  machine  all  depend 
upon  the  rigger.  Consider  that  while  the  engine  may  fail,  the 
pilot  may  still  glide  safely  to  earth;  but  if  the  airplane  fails,  then 
all  is  lost.  The  responsibility  of  the  rigger  is  therefore  very  great, 
and  he  should  strive  to  become  a  sound  and  reliable  expert  on  all 
matters  relating  to  his  art.  For  an  art  it  is,  and  one  bound  to  be- 
come increasingly  important  as  time  passes. 

GENERAL    RULES    KOH    RIGGING. 

There  are  two  kinds  oi  machines — the  tractor  and  the  pusher. 
The  pusher  type  is  a  type  which  is  now  dying  out.  The  principles 
of  rigging  for  both  are  the  same.  The  different  steps  of  rigging  are 
as  follows: 

1.  Get  a  blue  print  or  rigging  diagram  of  the  machine  and  look 
at  the  essential  measurements. 

2.  True  up  the  fuselage  and  lix  the  tanks  and  internal  fittings. 

3.  Put  on  the  undercarriage  (landing  gear),  in  oi'der  to  insure  that 
the  machine  can  not  fall . 

4.  Rig,  fix,  and  true  up  the  center  sections  of  the  main  planes. 

5.  Rig  the  main  planes  separately. 

6.  Attach  and  true  up  the  main  planes. 

7.  Rig  tail  (separately,  if  necessary).     Fix  and  true  up. 

8.  Attach  ])alancing  surfaces  and  adjust  controls. 

9.  Check  all  measurements  and  see  that  every  pin,  nut,  etc.,  is 
locked. 

10.  Put  engine  in  machine. 

11.  Look  over  the  whole  machine  to  see  if  everything  is  correct. 
Before  starting  work  on  a  machine,  get  into  overalls,  because  a 

man  can  not  do  proper  work  if  he  has  to  think  of  his  clothes.     See 
that  all  the  necessary  tools  are  handy.     Sort  and  lay  out  the  planes, 
4fiG4.V- 18 2 


18  AIR  SERVICE  HANDBOOK. 

struts,  cables,  etc.,  putting  each  more  or  less  in  its  relative  position 
(if  there  is  suflScient  room  in  the  shed). 
The  useful  tools  are  as  follows: 

1  side-cutting  pliers. 

1  round -nose  pliers. 

1  small  three-cornered  file  (to  ease  burrs  on  bolts  and  pins). 

1  hammer  (to  be  used  only  when  absolutely  necessary)  and 
copper  or  brass  drift. 

I  carpenter's  level. 

4  plumb  bobs. 

1  carpenter's  rule. 

1  straightedge  about  3  feet  long. 

1  long  and  1  short  tramel. 

1  steel  measuring  tape. 

1  ball  of  string. 

2  turnbuckle  keys. 

2  pairs  auto  combination  pliers,  not  for  nuts  or  bolts. 

Spanners  suitable  to  the  bolts  and  nuts  on  machine. 

End  wrenches  suitable  to  the  bolts  and  nuts  on  machine. 

See  that  split  cotter  pins,  nuts,  and  bolts,  etc.,  are  handy. 
Truing  up  the  fuselage. — In  factories  the  longerons  of  the  fuselage 
are  clamped  onto  a  table  which  has  blocks  on  it  shaped  as  required. 
In  the  field  the  rigging  has  to  be  done  by  measurement  from  the 
beginning.  It  is  unusual  in  the  field  for  the  squadrons  to  have  to 
rig  a  fuselage,  but  it  may  often  be  necessary  to  true  it  up.  Before 
attaching  any  wires  to  the  fuselage,  all  metal  fittings  should  be 
attached  in  the  proper  places  on  the  longerons.  All  struts  should  be 
fitted  in  their  sockets  in  order  to  prevent  delay  in  assembling.  The 
two  sides  of  the  fuselage  are  trued  up  first,  and  it  is  usual  either  to 
make  the  top  longeron  straight  or  to  make  the  whole  tail  symme- 
trical. This  must  be  found  out  from  the  blue  print.  When  each 
side  has  been  trued  up,  the  horizontal  compression  members  can  be 
placed  between  the  two  sides  and  the  bracing  and  the  internal 
cross  bracing  of  the  fuselage  can  be  adjusted.  While  doing  this 
the  fuselage  should  be  supported  on  two  trestles;  the  first  trestle 
should  be  toward  the  front  and  the  rear  trestle  about  two-thirds  of  the 
way  toward  the  rear.  This  causes  the  tail  to  stick  out  unsupported 
and  will  give  strains  on  the  fuselage  nearly  the  same  as  those  put 
on  the  ms  chine  in  flight .  Th  e  bracing  of  a  fuselage  is  done  by  means 
of  cables,  piano  wire,  or  special  tension  bars.  These  are  adjusted  in 
different  ways,  as  will  be  explained  later. 

The  engine  beds  are  usually  adjusted  permanently  as  far  as  we 
are  concerned,  and  it  is  only  necessary  to  see  that  the  remainder  of 
the  fuselage  is  trued  up  properly  with  respect  to  these. 


AIB  SEEVICE  HANDBOOK.  19 

When  the  fuselage  has  been  itself  trued  up,  it  is  then  necessary  to 
put  it  in  the  flying  position;  that  is,  the  engine  beds  must  be  hori- 
zontal and  the  horizontal  compression  members  should  be  also 
horizontal.  This  is  done  by  placing  a  straightedge  and  spirit  level 
on  the  engine  foundations,  and  you  must  be  very  careful  to  see  that 
the  bubble  is  exactly  in  the  center  of  the  level.  The  slightest  error 
will  be  much  magnified  toward  the  tail  and  wing  tips.  Great  care 
should  be  taken  to  block  the  machine  up  rigidly.  In  case  it  gets 
accidentally  disturbed  during  the  rigging  of  the  machine,  you  should 
constantly  verify  the  flying  position  by  running  the  straightedge  and 
spirit  level  over  the  engine  foundations.  Carefully  test  the  straight- 
edge before  using  it,  for,  being  usually  made  of  wood,  it  will  not  long 
remain  true.  Place  it  lightly  in  a  vise  and  in  such  a  position  that 
a  spirit  level  on  top  shows  the  bubble  exactly  in  the  center.  Now 
slowly  move  the  level  along  the  straightedge.  The  bubble  should 
remain  exactly  in  the  center.  If  it  does  not,  then  the  straightedge 
is  not  true  and  must  be  corrected.  Both  top  and  bottom  should  be 
true  and  exactly  the  same  distance  apart.  Never  omit  testing  the 
straightedge.     In  the  case  of  the  airplane  fitted  with  engines  of  the 


Fig.  2. 

rotary  type  the  "flying  position"  is  some  special  position  laid  down 
in  your  rigging  diagram  and  great  care  should  be  taken  to  secure 
accuracy. 

The  easiest  way  of  measuring  the  length  of  a  wire  is  by  means  of 
a  tramel.  This  is  a  piece  of  wood  which  carries  spikes  at  each  end — 
one  is  fixed,  and  the  other  is  adjustable. 

If  necessary,  the  wires  and  turnbuckles  should  now  be  locked  and 
painted. 

Put  the  tanks  in  the  machine  and  fix  all  the  internal  fittings. 
It  is  easy  to  get  at  the  inside  of  the  fuselage  now,  but  when  the 
wings  are  on  or  when  it  is  covered  with  dope  this  will  be  very  difficult. 

THE    PLANES. 

The  planes,  both  main,  center  section,  and  tail,  and  all  control 
surfaces  consist  of  spars  and  ribs  covered  with  dope.  If  the  siu-face 
ia  large,  they  are  braced  internally  with  wires,  cables,  or  compression 
members.  The  wings  consist  of  two  spars,  the  front  and  the  rear, 
which  are  kept  apart  by  compression  ribs  and  kept  in  shape  by 
bracing  wires,  which  are  fixed  to  the  ends  of  the  ribs,  diagonally 
opposite  and  sometimes  by  diagonal  ribs.  The  bracing  thus  consists 
of  a  number  of  rectangles,  usuallv  two  or  three  in  number.     To  true 


20 


AIR  SERVICE  HANDBOOK. 


up.  place  the  front  and  rear  spar  on  trestles  which  are  the  same  height . 
Attach  all  metal  fittings  to  the  spars.  Build  the  compression  ribs 
onto  the  spars,  and  fix  and  tighten  up  the  cross-bracing  wires.  Make 
certain  that  these  wires  are  of  the  same  length  by  means  of  the 
tramel.     Look  along  both  spars  and  see  that  they  are  straight. 

The  spars  are  always  made  of  ash  or  spruce,  usually  spruce.  The 
compression  ribs  are  usually  made  of  solid  spruce  or  some  such  ma- 
terial or  in  box  form.  They  are  sometines  steel  tubes.  They  fit 
into  sockets,  clipped  around  the  spar.  The  main  spars  should  not, 
as  a  rule,  be  drilled  to  take  fittings  as  they  are  thereby  weakened. 
On  no  account  should  a  spar  be  pierced  in  a  place  not  shown  on  the 
construction  diagram.  The  bracing  wires  are  attached  to  steel  clips 
called  wii-ing  plates  and  are  adjusted  by  turnbuckles. 

The  leading  edge  of  a  plane  is  not  meant  to  take  any  appreciable 
load  and  consists  of  some  liglit  wood  rounded  off  in  front.     The 


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Fig.  3. 

trailing  edge  has  no  thickness  and  consists  usually  of  a  thin  piece  of 
wood  or  wii'e  stretched  from  rib  to  rib  in  order  to  support  the  fabric. 
Form  ribs  run  from  the  leading  to  trailing  edge  at  intervals  of  about 
18  inches.  These  are  only  to  support  the  fabric  and  are  usually 
made  of  three-ply  wood.  Holes  are  bored  in  these  ribs  to  lighten 
them.  Between  the  leading  edge  and  the  front  spar  are  a  number 
of  light  form  ribs  to  enable  the  fabric  to  keep  its  shape.  In  some 
machines  these  ribs  are  replaced  by  a  layer  of  three-ply  wood.  A 
rib  consists  of  a  web  with  a  flange  on  the  top  and  bottom.  The 
fabric  is  tacked  onto  the  center  of  these  flanges  so  that  the  tack 
passes  down  the  web  of  the  rib.  Or  better  still,  the  fabric  is  sewn 
onto  these  ribs.  It  is  always  a  bad  practice  to  pierce  wood  if  there 
are  other  ways  of  doing  the  job.  \^Tien  the  wing  has  been  trued  up 
and  all  bolts,  etc.,  locked,  it  may  be  covered  with  fabric,  dope,  and 
varnish,  as  described  later.  All  metal  fittings  are  now  attached  to 
the  planes. 

The  center  section. — Place  the  center  section  struts  in  the  sockets 
on  the  center  section  and  attach  the  bracing  wires.  Lift  the  whole 
of  this  unit  and  place  the  bottom  of  the  struts  into  their  sockets  on 


AIR  SERVICE   HANDBOOK. 


21 


the  fuselage.  True  up  this  center  section.  The  machine  may  or 
may  not  have  stagger.  If  it  has  not,  the  front  center  section  struts 
will  be  vertically  over  the  struts  in  the  fuselage.  This  may  be  ad- 
justed by  dropping  a  string  with  pluml>  l)Gb  attached  from  the  wing 
attachment  of  the  front  spar,  seeing,  first  of  all,  if  the  pluml>  bob 
falls  immediately  over  that  of  the  lower  plane.  If  it  does  not.  alter 
the  incidence  wares.  Then  hold  the  string  in  front  of  the  center 
section  and  see  if  the  struts  are  upright.  If  they  are  not,  alter  cross- 
bracing  wires.  If  the  machine  has  stagger,  the  pluml)  bol)  must  fall 
a  certain  number  of  inches  in  front  or  behind  the  attachment  for  the 
lower  plane,  and  this  amount  will  be  found  from  the  blue  ])rint. 
In  some  machines  the  struts  are  splayed  outward  from  the  fuselage. 
In  any  case,  they  will  be  symmetrical  and  the  adjustments  can  be 
made  by  measuring  the  cross-bracing  wires,  and  seeing  that  they  are 
equal.  Make  certain  that  the  adjustment  of  the  center  section  is  correct, 
because  on  this  depends  the  whole  adjustment  of  the  machine. 


Fig.  4.  V 

The  icings. — Make  certain  that  the  fuselage  is  in  the  flying  posi- 
tion. Place  trestles  on  either  side  of  this  so  that  when  the  wings 
are  lifted  they  can  be  rested  on  these  trestles  in  approximately  the 
flying  position.  The  wings  on  each  side  of  the  machine  are  rigged 
separately.  This  is  done  by  supporting  the  planes  on  the  leading 
edges,  care  being  taken  that  the  leading  edge  is  not  damaged.  The 
struts  are  fitted  into  their  proper  sockets  and  the  l)racing  \vires  are 
adjusted  to  their  approximate  length  so  that  the  whole  pair  of  planes 
may  be  lifted  as  a  unit.  Attach  the  mngs  to  the  center  section  and 
fuselage.  When  the  planes  are  on  each  side,  take  away  the  trestles 
and  allow  the  strain  to  come  onto  the  landing  wires. 

True  up  the  main  planes  1)}^  adjusting  these  landing  \vires-making 
certain  that  all  the  other  wires  are  sufficiently  loose  so  as  to  take  no 
strain.  The  wings  should  be  symmetrical,  the  corresponding  wires 
on  either  side  being  equal.  The  incidence  should  be  that  shown  in 
the  blue  print  and  the  dihedral  angle  is  shown  there  also. 

To  measure  the  incidence. — One  method  of  finding  the  angle  of 
incidence  is  as  follows: 

Take  the  straightedge  and  test  it.  Place  one  corner  of  the  straight- 
edge underneath  and  against  the  center  of  the  rear  spar,  holding  it  in 
line  with  the  ribs.     Put  a  level  on  the  end  which  sticks  out  from  the 


82 


AIB  SERVICE  HANDBOOK. 


plane  and  hold  the  straightedge  horizontal.  Measure  from  the 
straightedge  to  the  center  of  the  bottom  of  the  front  spar  or  to  the 
lowest  part  of  the  leading  edge.  Make  the  measurement  that  shown 
in  the  blue  print  by  altering  the  landing  wiies.  See  that  the  spars 
are  straight  while  this  is  being  done.  Check  the  measurements 
under  each  set  of  struts.  If  the  machine  is  being  adjusted  after 
having  been  rigged,  slacken  off  all  wires  going  to  the  top  of  the  strut 
concerned  and  then  tighten  all  wires  going  to  the  bottom,  or  vice 


Fig.  5. 

versa.  Do  not  attempt  to  secure  this  adjustment  by  merely  altering 
the  incidence  wires.  This  latter  is  a  very  bad  practice  indeed,  and 
while,  owing  to  the  airplane  being  of  such  flimsy  construction,  it  may 
be  possible  to  change  the  angle  of  incidence  by  adjusting  merely  the 
incidence  wires,  the  result  of  such  practice  is  to  throw  other  wires 
into  undue  tension,  which  will  cause  the  framework  to  become  dis- 
torted. 

The  dihedral  angle. — One  method  of  securing  the  dihedral  angle, 
which  is  the  upward  inclination  of  the  wings  toward  their  tip,  is  as 


c 

\     / 

/\ 

\ 

B 

A 

E 

E'                       « 

3 

Fig.  6. 

follows,  and  this  method  will  at  the  same  time  give  you  the  angle 
of  incidence: 

Throw  a  string  over  the  top  of  both  spars  of  the  wings.  Keep  the 
string  tight  by  attaching  a  weight  or  by  attaching  it  to  some  heavy 
object.  The  strings  should  touch  the  wings  at  points  just  inside  the 
top  of  the  outer  struts.  The  measurement  taken  from  the  blue  print 
is  then  from  the  string  to  the  top  of  the  center  section  near  the  center 
section  struts.  This  measurement  should  be  taken  near  the  struts 
and  no  attempt  should  be  made  to  take  the  set  measurement  near  the 
center  of  the  center  section.  The  wings  on  each  side  should  be  sym- 
metrical, and  this  is  insured  by  making  the  landing  wires  in  corre- 


AIK  SERVICE  HANDBOOK. 


23 


eponding  bays  equal.  The  spars  should  be  kept  straight.  Sometimes 
the  diagonal  measurements  are  taken  from  the  bottom  of  one  strut  to 
the  top  of  another,  but  this  is  wrong  on  account  of  possible  inaccu- 
racies due  to  faulty  manufacture.  The  points  between  which  the 
diagonal  measurements  are  taken  must  be  at  fixed  distances  from  the 
butt  of  the  spars.  Such  distances  being  exactly  the  same  on  each 
Bide  of  the  machine,  thus: 


Fig.  7. 

It  would  be  better  still  to  use  the  center  line  of  the  fuselage  instead 
of  the  butt  of  the  spars  but  for  the  fact  that  t^uch  a  method  is  a  trouble- 
some one. 

Another  method  of  securing  the  diliedral  angle  and  also  the  angle 
of  incidence  is  by  means  of  the  dihedral  board.  The  dihedral  board 
is  a  light,  handy  thing  to  use  but  leads  to  many  errors  and  should  not 
be  used  unless  necessary.    The  reasons  are  as  follows:  The  dihedral 


board  is  probably  not  true.  If  you  must  use  it,  then  be  very  careful 
to  test  it  for  truth  beforehand .  Another  reason  against  its  use  is  that 
you  have  to  use  it  on  the  spars  between  the  struts,  and  that^is  just 
where  the  spars  will  have  a  little  permanent  set  up  or  down  which 
will,  of  course,  throw  out  the  accuracy  of  the  adjustment.  Then, 
again,  there  may  be  inequalities  of  surface  on  the  spar  due  to  faulty 
manufacture.    The  method  of  using  it  is  as  follows: 

If  the  dihedral  board  is  used,  then  the  bays  must  be  carefully 
measured  diagonally  as  explained  above.  Whichever  method  is 
UBed,  be  sure  that  after  the  job  is  done  the  spars  are  perfectly  straight. 


24 


AIR   SERVICE   HANDBOOK. 


Stagger. — The  stagger  is  the  distance  the  top  plane  is  in  advance 
of  the  bottom  plane  when  the  machine  is  in  the  fl>'ing  position.  The 
set  measurement  is  obtained  as  follows: 

The  plumb  lines  must  be  dropped  over  the  leading  edges  wherever 
struts  occur  and  also  near  the  fuselage.  The  set  measurement  is 
taken  from  the  front  of  the  lower  leading  edge  to  the  plumb  line. 
Remember  that  it  makes  a  difference  whether  you  measure  along  a 
horizontal  line  (which  can  be  found  by  using  a  straightedge  and 
spirit  level)  or  along  a  projection  of  the  chord.  The  correct  line 
along  which  to  measure  is  laid  down  in  your  rigging  diagram .     If  you 


make  a  mistake  and  measure  along  the  wrong  line,  this  may  make  a 
difference  of  a  quarter  of  an  inch  or  more  to  the  stagger,  with  the 
certain  result  that  the  airplane  will  be  nose  or  tail  heavy.  If  the 
stagger  was  put  correctly  on  the  center  section  in  the  first  instance, 
it  should  be  correct  when  the  main  planes  are  affixed. 

Now  adjust  the  drift  and  antidrift  wires. 

When  the  adjustment  of  the  angles  of  incidence,  dihedral  angle, 
and  stagger  have  been  secured,  the  incidence  wires  and  the  flying 
wires  should  be  tightened.  A\'Tien  this  has  been  done,  run  over  all 
your  measurements  again,  as  these  last  adjustments  may  possibly 
have  thrown  out  your  original  ones. 

Over-all  adjustments. — The  following  o\'er-all  measurements  should 
now  be  taken: 

The  straight  lines  "A"  and  "B"  must  be  equal.  The  point 
"C"  is  the  center  of  the  propeller  thrust.     The  points    'D"  and 


AIR   SERVICE   HANDBOOK.  26 

"  ■  E  "  are  marked  on  the  main  spar  and  must  in  each  case  be  the  same 
distance  from  the  butt  of  the  spar.  Do  not  attempt  to  make  "D" 
and  "E"  merely  the  sockets  on  the  outer  struts,  as  they  may  not 
have  been  placed  quite  accurately  by  the  manufacturers.  The  lines 
"A"  and  "B"  must  be  taken  from  both  top  and  bottom  spars — true 
measurements  on  each  side  of  the  airi)l9,ne.  Now  measure  the  dis- 
tance between  "F"  and  "G"  and  "H"  and  "G."  These  two 
measurements  must  be  equal.  "G"  is  the  center  of  the  fuselage  or 
rudder  post.  "F"  and  "H"  are  points  marked  on  the  top  and 
bottom  rear  spars,  the  same  distance  from  the  butt,  as  was  done 
before.  If  these  over-all  measurements  are  not  correct,  then  it  is 
probably  due  to  some  of  your  drift  or  antidrift  wires  being  too  tight 
or  too  slack.  It  may  possibly  be  due  to  the  fuselage  being  out  of 
true,  but,  of  course,  you  should  have  made  quite  sure  that  the  fuse- 
lage was  true  before  rigging  the  rest  of  the  machine.  Again,  it  may 
be  due  to  the  internal  bracing  wires  not  being  accurately  adjusted; 
but,  again,  that  should  have  been  done  before  covering  the  plane 
with  fabric. 

The  tail. — The  tail  may  be  either  an  adjustable  tail  or  a  fixed 
one.  The  angle  of  incidence  or  the  mean  angle  is  given  in  the 
rigging  diagram.  If  the  tail  is  adjustable,  see  that  the  control  is  in 
the  center  before  attaching  the  tail  plane.  To  true  up,  see  that  the 
spars  are  horizontal.  If  they  are  tapered  spars,  see  that  their  center 
lines  are  horizontal.  The  spars  should  be  straight  and  the  corre- 
sponding bracing  wires  on  either  side  should  be  equal  and  should 
bear  equal  strains.  Verify  the  position  of  the  tail  plane  by  stand- 
ing behind  the  machine  and  seeing  that  it  is  symmetrical  with  the 
center  of  the  main  planes.  In  some  machines  there  is  an  adjustment 
for  changing  the  ang  e  of  incidence  of  the  stabilizer.  The  greatest 
care  should  be  taken  when  increasing  the  angle  of  incidence  on  the 
tail.  Only  a  comparatively  small  increase  in  the  incidence  makes 
the  machine  dangerously  unstable,  as  explained  in  theory  of  flight. 
If  a  machine  is  nose  or  tail  heavy,  after  it  has  once  been  trued  up 
properly,  it  is  probably  due  to  the  fuselage  becoming  strained 
rather  than  a  wrong  angle  on  the  tail,  and  the  fuselage  should  be 
retrued  before  the  tail  is  touched. 

Control  surfaces. — Before  attaching  the  control  surfaces,  lash  the 
control  lever  and  the  rudder  bar  in  the  central  position.  The  pilot 
depends  entirely  on  these  control  surfaces  for  managing  the  plane,  so 
that  the  greatest  care  must  be  exercised  in  properly  adjusting  these 
surfaces.  When  the  surfaces  have  been  attached  to  the  planes,  never 
let  them  hang  down  without  support,  as  this  strains  the  hinges. 

The  ailerons  should  be  rigged  so  that  when  the  machine  is  in  flight 
they  are  in  a  fair,  true  line  with  the  surface  in  front  and  to  which 
they  are  hinged.     The  ailerons  are  hinged  to  the  main  planes  and 


26  AIR  SERVICE  HANDBOOK. 

are  then  attached  to  the  aileron  balance  cable  or  balance  springs. 
This  cable  should  be  adjusted  so  that  the  rear  edge  of  the  aileron  is 
1  inch  (may  alter  with  type  of  machine)  below  the  trailing  edge  of 
the  plane.  Connect  the  control  cables  to  the  ailerons.  Remember 
that  controlling  surfaces  must  never  be  adjusted  with  a  view  to 
altering  the  stability  of  the  machine.  Nothing  can  be  accomplished 
in  that  way.  The  only  result  will  be  that  the  control  of  the  air- 
plane will  be  spoiled.  If  the  ailerons  are  adjusted  too  high,  the 
machine  feels  "floppy."  If  the  ailerons  are  adjusted  too  low,  it 
makes  the  machine  unstable  and  tiring  to  fly.  In  both  cases  the 
machine  is  inefficiently  rigged. 

The  elevators,  like  the  ailerons,  should  set  fairly  behind  the  tail 
plane  when  the  machine  is  in  flight.  Because  the  controls  can  not 
be  adjusted  too  tightly,  the  elevators  also  must  hang  down  a  little  bit 
when  the  machine  is  at  rest.  They  should  be  adjusted  symmetric- 
ally on  either  side,  and  this  should  be  checked  by  eyo  as  well  as  by 
measurement. 

The  rudder  is  sometimes  set  at  a  small  angle  with  regard  to  the  cen- 
ter line  of  the  machine  in  order  to  help  the  adjustment  for  torque  of 
engine.  This  adjustment  should  be  checked  also  by  eye  and  meas- 
urement. 

Control  cables. — The  adjustment  of  control  cables  is  quite  an  art 
and  upon  it  will  depend,  to  a  large  degree,  the  quick  and  easy  con- 
trol of  the  airplane  by  the  pilot.  Having  rigged  the  controlling 
surfaces,  remove  the  lashing  which  has  kept  the  levers  in  the  central 
position.  Then,  sitting  in  the  pilot  seat,  move  the  control  levers 
smartly.  Tension  up  the  control  cables  so  that  when  the  levers 
are  smartly  moved  there  is  no  perceptible  snatch  or  lag.  Be  careful 
not  to  tension  up  the  cables  more  than  is  necessary  to  effect  this. 
If  you  tighten  the  control  cables  too  much,  they  will  bind  round  the 
pulleys  and  the  result  is  hard  work  for  the  pilot  and  it  also  throws 
dangerous  stresses  upon  the  controlling  surfaces,  which  are  some- 
times of  rather  light  construction.  It  will  also  cause  the  cables  to 
fray  round  the  pulleys  quicker  than  would  otherwise  be  the  case. 
Now,  having  tensioned  the  cables  sufficiently  to  take  out  the  snatch 
or  lag,  place  the  levers  in  their  neutral  position  and  move  them 
backwards  or  forwards  not  more  than  an  eighth  of  an  inch  either 
Bide  of  the  neutral  position.  If  the  adjustment  is  correct,  you 
should  be  able  to  see  the  controlling  surfaces  move.  If  they  do  not 
move,  the  control  cables  are  too  slack. 

Tail  skid. — The  tail  skid  is  usually  made  of  ash  and  is  stream  lined. 
Care  should  be  taken  that  the  tail-skid  spring  is  at  the  proper  ten- 
sion. If  it  is  too  loose  the  machine  may  jar  heavily  on  the  rudder- 
post,  and  this  will  strain  the  whole  fuselage.  A  safety  cable  should 
be  fitted  through  the  tail-skid  spring  to  prevent  the  front  end  stick- 


AIR  SERVICE  HANDBOOK.  27 

ing  into  the  ground  in  case  of  a  bad  landing.  If  the  skid  is  steerable 
it  is  controlled  by  cables  working  from  the  rudder  bar,  and  these 
controls  should  have  springs  on  them  to  prevent  sudden  jerks  coming 
on  the  rudder  bar  and  surface. 

The  landing  gear. — The  landing  gear  must  be  very  carefully  aligned 
as  laid  down  in  the  rigging  diagram. 

1.  Be  very  careful  to  see  that  the  landing  gear  struts  bed  down 
well  in  their  sockets.  If  this  is  not  done,  then  after  a  few  rough 
landings  they  will  bed  down  farther  and  throw  the  landing  gear 
out  of  alignment,  with  the  result  that  the  machine  will  not  taxi 
straight. 

2.  When  rigging  the  landing  gear,  the  airplane  must  be  blocked 
up  in  its  flying  position,  and  sufficiently  high  so  that  the  wheels  are 
off  the  ground.    When  in  this  position  the  axle  must  be  horizontal. 

3.  Be  very  careful  to  see  that  shock  absorbers  are  of  equal  tension 
and  that  the  same  length  of  elastic  cord  and  the  same  number  of 
turns  are  used  in  each  absorber. 


:^y'r'^ 

FlC.  10. 

4.  Errors  in  the  fore-and-aft  adjustment  of  the  axle  may  make  the 
machine  unstable  when  landing;  and  the  machine  may  either  pitch 
onto  the  propeller  or  break  the  tail  at  the  moment  of  landing  if  the 
adjustment  is  not  correct. 

Covering  the  fuselage. — When  covering  the  fuselage,  start  from  the 
front  and  top  and  work  backward,  so  that  there  will  be  no  hole  near 
the  engine  to  catch  oil.  Laco  the  fabric  fairly  tight  so  as  to  make 
the  skin  friction  small.  If  there  are  any  overlaps,  make  them  so  as 
not  to  catch  the  wind. 

Allowance  for  torque  of  propeller. — This  may  be  taken  up  on  the 
wings  or  by  the  rudder  or  both.  To  adjust  the  wings  it  will  be  necessary 
to  increase  the  angle  atthe  tip  of  one  plane  and  decrease  it  an  equal 
amount  on  the  other.  That  is,  give  the  planes  "wash  out"  or 
"wash  in."  If  the  propeller  rotates  right-handed  in  a  tractor,  it 
will  be  necessary  to  increase  the  angle  on  the  left  main  plane.  A 
tractor  machine  with  a  right-handed  propeller  will  also  require  a 
little  right  rudder.  In  some  machines  it  has  been  customary  to  take 
the  allowance  for  torque  on  the  ailerons,  but  this  is  a  bad  practice. 

Spars  and  struts. — All  spars  and  struts  must  be  perfectly  straight. 

The  above  diagram  shows  a  section  tlirough  an  interplane  strut. 
If  it  is  to  be  prevented  from  bending,  then  the  stress  of  compression 


28 


AIR  SERVICE  HANDBOOK. 


must  be  equally  disposed  round  the  center  of  strength.  If  it  is  not 
straight,  there  will  be  more  compression  on  one  side  of  the  center 
of  strength  than  on  the  other  side,  in  which  case  the  strut  will  be 
forced  to  take  a  bending  stress  for  which  it  was  not  designed.  Even 
if  it  does  not  break  it  will  in  effect  become  shorter,  and  thus  throw 
out  of  adjustment  all  the  wires  attached  to  the  top  and  bottom  of  it, 
with  the  result  that  the  flight  efficiency  of  the  airplane  will  be 


y 


T^ 


.jU-^f 


^>.i-5r''*r-w. 


Fig.  11. 


spoiled.     Besides,  an  undue  and  dangerous  stress  is  being  thrown 
upon  other  wires. 

1.  Where  spars  are  concerned,  there  is  an  exception  known  as  the 
arch.  For  instance,  in  the  case  of  the  Maurice  Farman,  the  spars  of 
the  center  section  plane,  which  have  to  take  the  weight  of  the  nacelle, 
are  arched  upward.  If  this  was  not  done,  it  is  possible  that  rough 
landings  might  result  in  the  weight  of  the  nacelle,  causing  the  spars 


*  t>%.^ 


4.U    ^ov^xcL 


to  bend  down  a  little.  This  would  produce  a  dangerous  bending 
stress,  but  as  long  as  the  wood  is  arched,  or  at  any  rate  kept  from 
bending  downward,  it  will  remain  in  direct  compression  and  no 
danger  can  result. 

2.  Struts  and  spars  must  be  symmetrical;  by  that  I  mean  that  the 
cross-sectional  dimension  must  be  correct,  as  otherwise  there  will 
be  bulging  places  on  the  outside,  with  the  result  that  the  stress  will 
not  be  evenly  disposed  around  the  center  of  strength  and  the  bending 
stress  will  be  produced. 

3.  Struts,  spars,  etc.,  must  be  properly  bedded  into  their  sockets 
or  fittings.     To  begin  with,  they  must  be  a  good  pushing  or  gentle 


AIR  SERVICE   HANDBOOK.  29 

tapping  fit.  They  Jinist  never  be  driven  with  a  heavy  hammer. 
If  the  sockets  do  not  fit,  it  is  better  for  them  to  be  too  large  than  too 
small.  Again,  spars  and  stmts  must  bed  well  down  all  over  their 
cross-sectional  area;  otherwise  the  stress  of  compression  will  be  taken 
on  one  part  of  the  cross-sectional  area,  with  the  result  that  it  will  not 
be  evenly  disposed  around  the  center  of  strength,  and  that  will 
produce  a  bending  stress.  The  bottom  of  struts  or  spars  should  be 
covered  with  some  sort  of  paint,  bedded  into  the  socket  or  fitting 
and  then  withdrawn,  to  see  if  the  paint  has  stuck  all  over  the  bottom 
of  the  fitting. 

4.  Do  not  trust  to  the  angle  of  the  socket  being  correct  when  the 
niacliine  is  rigged  for  the  first  time;  and,  as  the  planes  are  being 
adjusted,  keep  an  eye  on  all  sockets  to  insure  that  the  edges  do  no 
damage  the  wood  fibers. 

5.  The  atmosphere  is  sometimes  much  damper  at  one  time  than 
another,  and  this  causes  the  wood  to  expand  and  contract  appreciably . 


Fig.  13. 

This  would  not  matter  but  for  the  fact  that  it  does  not  expand  and 
contract  uniformly  but  becomes  unsymmetrical  or  distorted.  This 
should  be  minimized  by  varnishing  the  wood  well  to  keep  out  the 
moisture.  This  can  be  done  with  airplane  dope,  which  is  very  good 
for  the  purpose. 

6.  Sometimes,  for  lightness,  a  fitting  is  bolted  onto  the  end  of  a 
strut,  and  fabric  is  wound  round  the  end  to  prevent  the  strut  splitting. 

The  funclion  of  interplmie  struls. — These  struts  have  to  keep  the 
planes  apart  and  they  must  also  keep  them  in  their  correct  attitude. 
This  is  only  so  when  the  spars  of  the  bottom  plane  are  parallel  to 
those  of  the  top.  The  chord  of  the  top  plane  must  also  be  parallel 
with  the  chord  of  the  bottom  plane.  If  that  is  not  so,  then  one  plane 
will  not  have  the  same  angle  of  incidence  as  the  other.  You  may 
think  that  all  you  have  to  do  is  to  cut  all  your  struts  of  the  same 
length,  but  that  is  not  the  case. 

Sometimes,  as  illustrated  in  the  diagram,  the  rear  spar  is  not  as 
thick  as  the  main  spar.  It  is  then  necessary  to  make  up  for  the  lack 
of  thickness  by  making  the  rear  struts  correspondingly  longer.  If 
that  is  not  done,  the  top  and  bottom  chords  will  not  be  parallel  and 


80  AIB  SEKVICE  HANDBOOK 

the  top  and  bottom  planes  will  have  different  angles  of  incidence. 
Also,  the  sockets  or  fittings  or  even  spars  upon  which  they  are  placed 
sometimes  vary  in  thickness  and  this  must  be  offset  by  altering  the 
length  of  struts.  The  proper  way  to  proceed  in  order  to  make  sure 
that  everything  is  right  is  to  measure  the  distance  between  the  top 
and  bottom  spars  on  each  side  of  each  strut,  and  if  that  distance  or 
"gap,"  as  it  is  called,  is  not  as  specified  in  your  rigging  diagram,  make 
it  correct  by  changing  the  length  of  your  strut.  "WTien  measuring 
the  gap  between  the  top  and  bottom  spars,  always  be  careful  to 
measure  from  the  center  of  the  spar,  as  it  may  be  set  at  an  angle  and 
the  rear  of  the  spar  may  be  considerably  lower  than  its  front. 

Wires. — The  following  points  must  be  carefully  observed  where 
wire  is  concerned: 

1.  Quality:  It  must  not  be  too  hard  or  too  soft.  An  easy  practical 
way  of  learning  to  know  the  quality  of  wire  is  as  follows :  Take  three 
pieces  of  wire  all  of  the  same  gauge,  and  each  about  a  foot  in  length; 
one  piece  should  be  too  soft,  another  piece  should  be  too  hard,  and 
the  third  piece  of  the  right  quality.  Fix  them  in  a  vise  about  an 
inch  apart  and  in  a  vertical  position,  and  with  the  light  from  a  ^vin- 
dow  shining  upon  them.  Burnish  them,  if  necessary,  and  you  will 
see  a  bar  of  light  reflected  from  each  wire.  Now  bend  the  wires 
over  as  far  as  possible;  where  the  soft  wire  is  concerned,  it  will  squash 
out  at  the  bend  and  you  will  see  this  because  the  bend  of  light  will 
have  broadened  out  there.  In  the  case  of  the  wire  which  is  too  hard, 
the  bend  of  light  will  be  broadened  out  very  little  at  the  turn,  but 
if  you  look  carefully  you  will  see  some  little  cracks  or  roughnesses 
on  the  surface.  In  the  case  of  the  wire  of  the  right  quality,  the  bend 
of  light  may  have  broadened  out  a  very  little  at  the  tiu^n,  but  there 
will  be  no  cracks  or  roughnesses  in  it  at  all.  By  making  this  experi- 
ment two  or  three  times,  you  will  soon  learn  to  know  good  wire  from 
bad  and  also  learn  to  know  strength  of  hand  necessary  to  bend  the 
right  quality. 

2.  Wire  must  not  be  damaged;  that  is  to  say,  it  must  be  unkinked, 
rustless,  and  imscored. 

3.  As  regards  keeping  vnxe  in  good  condition,  where  the  outside 
wires  are  concerned,  they  should  be  kept  well  greased  or  oiled, 
especially  where  bent  over  at  the  ends.  This  does  not  mean  that 
large  bits  of  grease  must  be  left  on  the  wii-es,  simply  that  there  should 
be  a  film  of  oil.  In  the  case  of  internal  bracing  -n-ires,  which  can  not 
be  reached  for  the  purpose  of  regreasing  them,  you  will  prevent  them 
from  rusting  by  painting  them  with  white-lead  paint.  You  must  be 
very  careful  to  see  that  the  wire  is  perfectly  clean  and  dry  before 
painting  with  white-lead  paint.  A  greasy  finger  mark  is  sufficient 
to  stop  the  paint  from  sticking  to  the  wire.     In  such  a  case,  there  will 


ATR  SERVICE  HANDBOOK.  81 

be  a  little  space  between  the  paint  and  the  wnre.  Air  can  enter 
there  and  cause  the  wire  to  rust  under  the  paint.  The  paint  should 
be  of  a  light  color  so  as  to  show  any  signs  of  rust. 

4.  Wires  and  cables  may  be  stream  lined  by  binding  onto  them  a 
V-shaped  faring  made  of  spruce.  The  wire  and  faring  can  be  then 
painted  to  decrease  the  resistance.  For  single  wires,  this  is  not  much 
of  a  gain,  as  a  semicircle  or  triangle  is  a  bad  form  of  stream  line.  For 
the  duplicated  flying  wires  it  is  an  advantage,  as  it  prevents  them 
vibrating  separately  and  thus  helps  to  decrease  the  head  resistance. 
In  any  case,  wires  where  they  cro.s3  should  be  joined  together  by 
threading  them  through  a  little  stream-line  fiber  washer. 

Tension  of  wires.— The  tension  to  which  you  adjust  the  wires  is  of 
the  greatest  importance.  All  the  wires  on  the  airplane  should  be  of 
the  same  tension,  otherwise  the  airplane  will  quickly  become  dis- 
torted and  fly  badly.  As  a  rule,  the  wires  are  tensioned  too  much. 
The  tension  should  be  sufficient  to  keep  the  framework  rigid.  Any- 
thing more  than  that  spoils  the  factor  of  safety,  throws  various  parts 
of  the  framework  into  undue  compression,  pulls  the  fittings  into  the 
wood,  and  will,  in  the  end,  distort  the  whole  framework  of  the  air- 
plane. Only  experience  will  tell  you  what  tension  to  employ  and 
assist  you  in  making  all  the  wires  the  same  tension.  Learn  the  con- 
struction of  various  types  of  airplanes,  the  work  the  various  parts  do, 
and  cultivate  a  touch  for  tensioning  wires  b)'  constantly  handling 
them.  While  at  rest  the  landing  wires  will  bear  more  strain  than  the 
flj'ing  wii'es  on  account  of  the  weight  of  the  plane.  The  opposite 
happens  when  the  machine  is  in  the  air.  If  the  fl\ing  \^'ii'es  are  trued 
up  slackly,  the  whole  rigging  of  the  airplane  alters  directly  the  ma- 
chine gets  into  the  air.  In  some  cases  you  will  find  wires  having  no 
opposing  wires  pulling  in  the  opposite  direction.  In  such  cases, 
be  extremely  careful  not  to  tighten  such  wires  beyond  taking  up  the 
slack.  If  care  is  not  taken,  the  incidence  of  the  plane  will  be 
changed,  resulting  in  change  of  both  lift  and  drift  at  that  part  of  the 
plane.  Such  a  condition  will  cause  the  machine  to  lose  its  direc- 
tional stability  and  also  to  fly  one  wing  down.  I  can  not  impress 
this  matter  of  tension  upon  you  too  strongly.  It  is  of  the  utmost 
importance.  W^hen  you  have  learned  this  and  also  learned  to  be 
accurate  in  getting  the  various  adjustments  you  are  on  the  way  to 
becoming  a  good  rigger. 

Wire  loops. — Wire  is  often  bent  over  at  the  end  in  the  form  of  a 
loop.  These  loops,  even  when  made  perfectly,  have  a  tendency  to 
elongate,  thus  spoiling  the  adjustment  of  the  wire.  Great  care 
should  be  taken  to  minimize  this  as  much  as  possible.  The  rules  to 
be  observed  are  as  follows: 


32 


AIR  SERVICE  HANDBOOK. 


1.  The  size  of  the  loop  should  be  as  small  as  possible  within  reason. 
By  that  I  mean  that  it  should  not  be  so  small  as  to  create  the  possi- 
bility of  the  wire  breaking. 

2.  The  shape  of  the  loop  must  be  symmetrical. 

3.  The  loop  should  have  good  shoulders  in  order  to  prevent  the 
ferrule  from  slipping  \vp.  At  the  same  time  the  shoulders  should 
have  no  angular  points. 

5.  The  ferrule  should  fit  the  cable;  if  it  is  too  large  the  loop  will 
slip  anyhow. 

6.  When  the  loo])  is  finished  it  should  be  undamaged  and  should 
not  be,  as  is  often  the  case,  badly  scored. 

7.  Wires  up  to  12  gauge  should  be  bent  easily  by  hand  with  the 
aid  of  a  round-nose  plier.  The  bending  should  be  done  firmly  and 
quickly.     This  is  quite  a  knack  with  the  larger  sizes  of  wire. 


«-»»der    SC'o-V 


Fig.  14, 


Stranded  wire  cables. — There  are  two  kinds  of  cable  used  in  rigging- 
airplanes,  one  of  which  is  much  harder  than  the  other.  The  loops 
on  the  first  are  usually  made  by  serving  the  cable  with  wire  and  then 
soldering.  Loops  on  the  latter,  the  softer  wire,  may  be  made  by 
splicing.  When  serving  the  cable  with  wire  the  winding  must  be 
even,  with  a  nice  stream-line  effect  at  the  end  of  the  winding.  When 
solder  is  used  care  must  be  taken  that  the  flux  does  not  go  beyond 
soldered  portion  of  cable.  Only  nonacidfux  should  be  used  in  solder- 
ing. The  length  of  the  served  portion  should  be  at  least  fifteen  times 
the  diameter  of  the  cable  as  shown  in  diagram. 

If  the  cable  is  spliced,  every  strand  must  take  its  proper  share  of 
the  strain.  Sharp  turns  should  be  avoided.  When  hammering 
the  splice,  a  sharp  or  too  hard  an  instrument  should  not  be  used,  as 
this  is  liable  to  injure  the  strand.  No  splice  should  be  served  with 
twine  until  it  has  been  inspected  and  passed  by  whoever  is  in  charge 
of  the  shop.     Only  the  very  end  of  the  splice  should  be  served,  asthia 


AIR  SERVICE  HANDBOOK.  88 

is  only  intended  to  prevent  the  short  ends  of  wire  from  coming  away 
from  the  strand.  Stranded  cable  when  overstrained  nearly  always 
breaks  just  above  the  splice.  Thimbles  of  soft  metal  should  always 
be  used  with  stranded  cable.  Should  a  strand  become  broken,  then 
the  cable  must  l)e  replaced  by  another.  ( 'ontrol  cables  have  a  way  of 
wearinf?  oul  and  frayinjj;  ^^  henever  ihey  pass  over  the  pulley.  Every 
time  an  airplane  comes  down  from  a  fli2;ht  the  rifi;ger  .'ihould  carefully 
examine  the  cables  wherever  they  pa.ss  round  pulleys,  and  if  he  finds 
a  strand  broken  he  should  rejjort  the  fact  at  once.  The  aileron  bal- 
ance wire  on  top  of  the  top  ])lane  is  often  forgotten,  since  it  is  necessary 
to  fetch  a  high  pair  of  steps  in  order  to  examine  it.  Do  not  neglect 
this.  Both  wires  and  cables  are  liable  to  be  damaged  where  they 
cross;  to  prevent  this,  a  little  block  of  fiber,  stream-lined,  is  threaded 
on  to  the  cables  so  as  to  prevent  them  touching.  The  wires  of  the 
interior  of  the  machine  may  be  wrapped  with  adhesive  tape  but  as 
this  collects  moi.sture  it  is  not  a  good  thing  to  use  this  tape  on  those 
cables  exposed  to  th(>  air. 


^u-^^^-^MMummimmimm 


Fig.  lo. 

All  the  cables  should  be  stretched  before  fitting  and  should  l)e  well 
greased  where  they  run  through  pulleys  or  fair-leads.  Fair-leads 
should  be  made  of  rawhide,  rather  than  of  metal,  as  rawhide  gives 
less  wear.  When  a  cable  is  being  inspected  to  see  if  it  is  frayed,  all 
the  old  oil  must  be  wiped  off  and  the  fingers  should  be  gently  run 
over  the  suspected  place.  Any  broken  strands  will  be  easily  felt. 
Sometimes  the  inner  strands  are  broken,  and  this  may  l)e  found  out 
by  very  gently  bending  the  cables  liackward  and  forward. 

Cables  may  be  cut  by  heating  (juickly  in  a  l)low  torch  flame. 
This  makes  the  wire  soft  and  also  keeps  the  ends  from  fraying.  Care 
must  be  taken  not  to  let  the  heat  travel  far  down  the  cable,  and  this 
can  be  done  by  holding  cable  in  metal  to  conduct  away  the  heat. 

Stream-line  wires. — Stream -line  wires  are  made  by  rolling  steel 
rods.  They  are  screw  threaded  at  either  end  with  a  left  and  right 
handed  thread.  These  rods  are  cut  to  length  for  each  machine  so 
that  if  one  breaks  a  special  rod  has  to  be  obtained  in  order  to  rei)lace 
it.  The  ends  of  these  rods  fit  into  special  Y-shaped  fittings,  which 
are  also  left  and  right  hand  threaded  to  fit  onto  the  ends  of  the  rod. 
The  rods  also  carry  a  locking  nut  at  each  end  so  that  when  the 
46648—18 3 


34  AIR  SERVICE   HANDBOOK. 

required  adjustment  is  made,  the  wires  can  be  fixed  there.  The  end 
fittings  are  pinned  to  the  wiring  plates  on  the  planes  l)y  means  of 
special  steel  pins.  There  is  a  small  hole,  aljout  halfway  down  each 
fitting,  and  to  be  safe  the  end  of  the  wire  must  pass  this  hole.  There 
is  another  kind  of  fitting  in  which  the  wire  is  screwed  through  a  little 
metal  rod.  This  latter  is  a  l)etter  method,  as  it  allows  the  wii-es  to 
vibrate  without  putting  any  side  strain  on  them.  The  wires  must 
lie  in  the  flow  of  air  in  the  same  manner  as  do  the  struts.  By  using 
stream-line  wires  the  lift  of  the  machine  is  increased  appreciably 
and  the  speed  a  little.  When  stream -line  wires  are  overstrained  they 
tend  to  draw  out  just  above  the  screw-threaded  portion,  and  this  can 
easily  be  felt  by  running  the  fingers  down  the  edge  of  the  wdre  and 
can  be  seen.  Stream-line  wires  should  he  kept  bright  and  slightly 
oiled. 

Tension  rods. — These  are  rods  used  in  the  construction  of  many 
fuselages,  and  the  same  rules  apply  to  them  as  to  stream-line  wires, 
except  that  they  are  round  and  can  !)e  locked  in  any  position. 


^  Turns  anc^  pu// 
Jiyh  ^<3nd  Af&at, 

Yui.  Itj. 

TunibucHes. — A  turnbuckle  is  composed  of  a  central  barrel,  into 
each  end  of  which  is  screwed  an  eyebolt.  The  bolts  at  either  end  are 
screw  threaded  left  and  right  handed.  Wires  are  taken  from  the 
ends  of  the  eyel)olts,  and  so  l>y  turning  the  barrel  the  wires  can  be 
adjusted  to  their  proper  tension.  Eyel>olts  must  be  a  good  fit  in  the 
barrel;  that  is  to  say.  not  slack  and  not  very  tight.  There  is  a  rule 
that  the  eyebolts  must  V)e  screwed  into  the  l)arrel  for  a  distance  of 
not  less  than  thrice  their  diameter,  liut  it  is  better  to  screw  them  in  a 
good  deal  more  than  that.  If  the  eyebolt  is  screw  threaded  for  only 
a  short  distance,  the  bolts  should  be  screwed  into  the  barrel  till  the 
last  thread  is  flush  with  the  end  of  the  barrel  unless  otherwise  stated. 
The  turnbuckle  should  not  be  tightened  so  that  the  ends  of  the  eye- 
bolts  meet  in  the  middle.  If  this  happens,  new  cables  must  l)e 
fitted.  Turnbuckles  are  chosen  of  a  size  corresponding  to  that  of  the 
cable  used  with  them.  The  l)arrel  of  the  turnbuckle  looks  solid  but 
is  really  hollowed  out  and  is  much  more  frail  than  it  appears.  For 
that  reason  it  must  not  Ite  turned  l»y  soi/.ing  it  with  pliers,  as  that 


AIR   SERVICE   HANDBOOK.  35 

may  distort  it  or  spoil  the  bore.  The  proper  method  Ls  to  pass  a 
piece  of  wire  through  the  hole  in  the  center  and  to  use  that  as  a  lever. 
The  eyebolts  may  be  prevented  from  turning  by  holding  them  on 
the  ends  of  another  piece  of  wire.  When  the  correct  adjustment  is 
obtained,  the  turnlmckle  must  l)e  locked  to  prevent  it  from  unscrew- 
ing. It  is  quite  possible  to  lock  the  turnbuckle  in  such  a  way  that 
it  allows  it  to  unscrew  a  quarter  or  half  turn,  and  that  will  throw  the 
wires  out  of  the  very  tine  adjustment  necessary.  The  proper  way  is 
to  use  the  locking  mres  in  such  a  way  as  to  oppose  the  tendency  of 
the  turnbuckle  to  unscrew,  as  is  shown  in  the  diagram. 

The  wire  used  for  locking  a  turnbuckle  is  hard  copjjer  wire  or 
soft  iron  wire.  Turnbuckles  on  internal  wires  must  be  well  greased 
and  served  round  with  adhesive  ta])e  after  they  have  been  locked. 
On  no  account  may  the  barrel  of  a  turnbuckle  be  sawed  off  short. 

In  case  of  a  forced  landing,  it  may  be  necessary  to  mend  the 
machine  from  materials  at  one's  disposal  locally,  and  it  is  useful 
to  know  a  little  about  the  materials  used  in  the  construction  of  a 
machine. 

.MET.M.. 

1.  There  should  be  no  signs  of  rust  or  Haws. 

2.  Only  bright  bolts  ai^d  nuts  should  be  employed.  In  idr])lane 
construction,  bolts,  nuts,  and  pins  are  made  of  special  steel  and 
those  obtained  locally  should  be  looked  on  with  susi)icion  as  thev 
are  almost  certain  to  be  too  weak. 

3.  Piano  wire  should  not  have  been  previously  l)ent  and  must 
be  free  from  kinks. 

4.  Stranded  wire  or  cable  should  be  regularly  twisted  and  not 
frayed  at  any  point. 

5.  Tubing  should  he  perfectly  straight  and  should  not  show 
signs  of  having  been  previously' bent  and  subsetiuontly  straightened. 
Tubing  is  ustially  welded  into  its  sockets,  but  if  there  is  much  vibra- 
tion tubes  should  l>e  attached  to  the  sockets  by  being  pinned  and 
then  soft  soldered.  In  case  an  axle  becomes  bent,  it  can  be  mended 
temporarily  by  being  straightened  and  by  having  a  wood  iiif-hi 
core  put  in.     This  should  bring  the  machine  home. 

6.  Threads  of  bolts,  nuts,  and  screws  should  be  clean  and  not 
worn  or  burred.  Make  certain  that  these  are  screw  threadcil  on 
the  same  system  and  that  they  have  the  same  numl)er  of  turns  to 
the  inch. 

7.  Strut  sockets  and  other  metal  fittings  should  not  be  l)eut  out 
of  their  original  shape.  Such  fittings  should  not  be  used  if  they 
show  signs  of  having  been  bent  and  subsetjuently  straightened. 


36  AIR   SERVICE   HANDBOOK. 

In  the  case  of  aluminum  sockets,  care  must  be  taken  that  there 
are  no  cracks,  especially  where  the  sockets  have  previously  been 
subjected  to  severe  strains.  Eyeplates  and  eyebolts  should  show- 
no  signs  of  wear  or  fracture.  Wiring  plates  can  be  replaced  tem- 
porarily by  those  made  from  mild  steel.  Allow  plenty  of  metal  so 
as  to  insure  the  plates  being  strong  enough.  This  again  must  be 
only  a  temporary  measure.  The  properties  of  iron  are  described 
under  "Engine  material. " 

8.  All  metal  fittings  should  bear  the  inspection  mark  before  being 
used  on  an  airplane.  Any  fitting  which  has  been  subjected  to  a 
strain  should  be  inspected  by  a  qualified  officer  or  returned  to  the 
salvage  section,  and  this  latter  applies  to  all  airplane  material. 

9.  No  bolt,  pin,  or  turnbuckle  should  be  used  if  it  has  been  bent. 


The  correct  wood  for  the  various  parts  of  an  airplane  must  be 
used.  The  wood  used  must  have  a  good  clear  grain,  with  no  cross 
grain,  knots,  or  shakes.  Such  blemishes  mean  that  the  wood  is  in 
some  places  weaker  than  in  other  places,  and,  if  it  has  a  tendency 
to  bend,  then  it  will  go  at  those  weak  points.  All  wood  must  be 
properly  seasoned.  Struts,  spars,  etc.,  must  be  straight  and  un- 
damaged. When  a  bending  stress  comes  on  one  of  these  members 
the  outside  fibers  of  the  wood  are  doing  by  far  the  most  work.  If 
these  get  bruised  or  scored,  then  the  strut  or  spar,  suffers  in  strength 
much  more  than  one  might  think  at  first  sight,  and,  if  it  ever  gets 
a  tendency  to  bend,  it  is  likely  to  go  at  that  point.  The  two  woods 
most  generally  used  in  airplane  construction  are  ash  and  spruce. 
Spruce  is  the  strongest  wood  for  weight  that  grows.  Ash  is  a  very 
strong  but  heavy  wood,  but  it  is  very  good  at  resisting  sudden  shocks 
and  will  bend  considerably  before  breaking.  As  a  general  rule, 
spruce  is  used  for  the  main  spars,  struts,  compression  ribs,  and 
the  flanges  of  form  ribs.  Ash  is  used  in  the  longerons,  in  the  under- 
carriage struts,  in  skids,  and  in  the  three-ply  webs  of  form  ribs. 
It  is  also  used  for  engine  bearers,  if  metal  is  not  employed.  No 
spar,  strut,  etc.  should  be  bored  in  any  place  where  a  hole  was  not 
designed.  If  any  tiring  is  to  be  fitted  to  a  spar,  it  should  be  clipped 
round  it  with  a  suitable  clip.  Holes  in  wood  should  be  of  a  size 
that  the  bolts  can  be  pushed  in,  or  at  any  rate  not  more  than  gently 
tapped  in.  Bolts  must  not  be  hammered  into  wood,  as  doing  this 
splits  the  wood.  On  the  other  hand,  a  bolt  must  not  be  slack  in  a 
hole,  as  it  works  sideways  and  may  thus  split  the  s])ar,  not  to  speak 
of  throwing  out  of  adjustment  the  wires  leading  from  the  lug  or 


AIR  SERVICE  HANDBOOK.  37 

socket  under  the  bolthead.  As  has  been  before  stated,  all  wood 
should  be  well  varnished  so  as  to  prevent  damp  creeping  in  and  caus- 
ing expansion  or  contraction  of  the  fibers.  In  case  of  emergency, 
where  the  proper  wood  is  not  obtainable,  make  certain  that  the  wood 
used  is  sufficiently  strong  to  carry  the  strain. 

Nature  of  wood  under  stress. — Wood  for  its  weight  takes  the  stress 
of  compression  best  of  all.  For  instance,  a  walking  stick  of  about 
half  a  pound  weight  will,  if  kept  perfectly  straight.  ])robably  stand 
up  to  a  compression  stress  of  a  ton  or  more  before  crushing,  whereas 
if  the  same  stick  is  put  under  a  bending  load  it  will  ])robably  collapse 
to  a  stre.ss  of  not  more  than  50  pounds.  That  is  a  very  great  difference, 
and  since  weight  is  of  the  greatest  importance  in  an  airi>lane  the 
wood  must  as  far  as  possible  be  kept  in  a  state  of  direct  compression. 

Splicing  wood. — In  the  case  of  a  fracture  occurring  in  a  solid  spar 
or  one  of  the  box  type  that  is  wide  enough  (2  inches)  it  is  often  possi- 
ble to  make  a  good  repair  by  scarfing  on  a  new  length.  The  scarf 
must  be  long  compared  to  the  dejjth  of  the  spar  and  the  two  pieces 
of  wood  forming  it  must  be  a  good  fit  onto  each  other.  After  fitting 
the  two  halves  of  the  scarf  together  they  must  be  well  glued  and 
then  clamped  together  till  the  glue  is  set.  The  joint  is  then  planed 
up  and  examined  to  see  that  it  is  a  close  one  and  does  not  have  a 
thick  layer  of  glue  between  the  two  thicknesses  of  wood.  The 
two  halves  of  the  scarf  are  then  bolted  together  as  an  additional  pre- 
caution, large  washers  being  employed  under  the  bolthead  and  nut 
so  as  to  prevent  them  from  cutting  into  the  wood  and  crushing  it 
when  tightening  the  nut.  A  waxed  whipcord  lashing  is  then  served 
round  the  joint,  each  turn  of  the  cord  being  securely  knotted  to 
prevent  it  coming  adrift.  The  cord  may  be  glued  finally  as  a  further 
precaution. 

STRESSES    AND    STRAINS. 

In  order  to  rig  a  machine  intelligently  it  is  necessary  to  have  a 
correct  idea  of  the  work  every  wire  and  every  part  of  the  airplane  is 


Fig.  17. 

doing.  The  work  the  part  is  doing  is  known  as  stress.  If  owing  to 
undue  stress  the  material  becomes  distorted  then  such  distortion 
is  known  as  strain. 


38  AIR  SERVICE  HANDBOOK. 

Compression. — The  simple  stress  of  compression  produces  a  crush- 
ing strain.     As  an  example  the  interplane  and  fusilage  struts. 

Tension. — The  simple  stress  of  tension  results  in  the  strain  of 
elongation.     As  an  example  all  the  wires. 

Bending.— The  compound  stress  of  bending  is  composed  of  both 
tension  and  compression.  Now  we  will  suppose  we  are  going  to 
bend  a  piece  of  wood.  Before  being  bent  it  will  have  the  following 
appearance: 

You  see  that  the  top  line,  the  bottom  line,  and  the  center  line  are 
all  of  the  same  length.  Now  we  will  bend  it  right  round  in  a  circle, 
thus 

The  center  line  is  still  the  same  length  as  it  was  before  being  bent, 
but  you  will  note  that  the  top  line  being  on  the  outside  of  the  circle 
must  now  be  longer  than  the  center  line.  That  can  only  be  due  to 
the  strain  of  elongation.     That  is  produced  by  the  stress  of  tension. 

So  you  see  that  the  wood  be- 
tween the  center  line  and  the 
line  on  the  outside  of  the  circle 
is  in  tension.  The  greatest  ten- 
sion is  on  the  outside  of  the 
circle  because  there  the  elonga- 
tion is  greatest. 

You  will  notice  that  the  line 
on  the  inside  of  the  circle  which 
before  being  bent  was  the  same  length  as  the  center  line  must  now 
be  shorter  because  it  is  nearest  to  the  center  of  the  circle.  That  can 
only  be  due  to  the  strain  of  crushing.  That  can  only  be  produced 
by  a  state  of  compression.  So  you  see  that  the  wood  between  the 
center  line  and  the  inside  line  is  in  compression  and  the  greatest 
compression  is  nearest  to  the  inside  of  the  circle  because  there  the 
crushing  effect,  i.  e.,  the  strain,  is  greatest. 

By  this  you  will  see  that  the  wood  near  the  center  line  is  doing  the 
least  work.  That  is  why  it  is  possible  to  hollow  out  the  center  of 
spars  and  struts  without  unduly  weakening  them.  In  this  way 
25  to  33  per  cent  of  the  weight  of  wood  in  an  airplane  is  saved. 

Shear. — Shear  stress  is  such  that  when  the  material  breaks  under 
it  one  part  slides  over  the  other.  As  an  example  the  locking  pins. 
Some  of  the  bolts  are  in  a  state  of  shear  stress  also  because  in  some 
cases  there  are  lugs  underneath  the  boltheads  from  which  wires  are 
taken.  Owing  to  the  tension  of  the  wire  the  lug  is  exerting  a  side- 
ways pull  on  the  bolt  and  trying  to  break  it  in  such  a  way  as  to  make 
one  part  of  it  slide  over  the  other. 

Torsion. — The  stress  of  torsion.  This  is  a  twisting  stress  composed 
of  compression,  tension,  and  shear  stress.  As  an  example  the  pro- 
peller shaft  and  crank  shaft  of  an  engine. 


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AIR   SERVICE   HANDBOOK.  39 

Washers. — Under  the  bolthcacl  and  also  under  the  nut  a  washer 
must  be  placed.  This  should  be  a  very  large  wa^sher  comj)ared  \vdth 
any  other  form  of  engineering.  This  is  to  disperse  the  stress  over  a 
large  area  of  wood;  otherwise  the  washer  may  be  pulled  into  the  wood 
and  weaken  it,  besides  jiossibly  throwing  out  of  adjustment  the 
wires  attached  to  the  bolt  or  fitting. 

Locking. — As  regards  locking  the  bolt.^!  if  split  pins  are  used  be 
sure  to  see  that  they  are  used  in  such  a  way  that  the  nut  can  not 
possibly  unscrew.  If  a  castellated  nut  is  used  the  pin  should  fit 
the  hole  in  the  bolt  so  that  it  will  not  be  sheared  by  the  edge  of  the 
nut.  If  a  plain  nut  is  used  washers  should  be  put  under  the  nut  so 
that  it  reaches  exactly  to  the  bottom  of  the  hole  in  the  bolt  or  the 
nut  may  be  filed  a  little  bit  to  allow  it  to  go  down  past  the  hole.  If 
a  bolt  is  locked  by  burring  over  the  end  a  heavy  hammer  must  not 
be  used  in  order  to  try  and  spread  the  whole  head  of  the  bolt.  That 
might  damage  the  woodwork  inside  the  plane.  Use  a  small,  light 
hammer  and  gentlj-  tap  around  the  edge  of  the  bolt  until  it  is  burred 
over.  All  bolts  and  nuts  should  be  locked  in  a  positive  manner. 
A  split-ring  washer  does  not  lock  a  nut  in  this  manner  and  should 
not  be  used  without  some  other  form  of  locking  as  a  rule. 

IIANDUNC    OF    .\lin'],.AM;.S. 

An  extraordinary  amount  of  damage  is  done  bj'  the  mishandling 
oif  airplanes  and  in  blocking  them  up  from  the  groimd  in  the  wrong 
way.     The  golden  rule  to  observe  is  "Produce  no  bending  stresses." 

1.  Remember  that  nearly  all  the  wood  of  an  airplane  is  designed 
to  take  stress  of  direct  compression  and  it  can  not  be  safely  bent. 
In  Vilocking  an  airplane  up  from  the  ground  the  packing  must  be 
used  in  such  a  way  as  to  come  underneath  the  interplane  struts  and 
the  fuselage  struts.  Soft  packing  should  always  l^e  i)la( cd  on  the 
})()ints  upon  which  the  airplane  rests. 

2.  When  pulling  the  m.achine  along  the  ground,  ahvays,  if  possible, 
pull  from  the  landing  gear.  If  it  is  necessary  to  pull  from  ekewhere 
do  so  by  grasping  the  interplane  struts  as  low  down  as  possible. 

3.  Never  lift  or  put  any  strain  on  the  leading  and  trailing  edges  of 
the  planes  and  do  not  cover  them  with  oily  finger  marks. 

4.  As  regards  handling  ])arts  of  air])lanes,  never  lay  anything 
covered  with  fal)ric  on  a  concrete  floor  as  any  slight  niovtUK-nt  will 
cause  the  fabric  to  scrape  over  the  concrete  with  resultant  damage. 

5.  Struts,  spars,  etc.,  should  never  be  left  about  the  floor,  as  in 
such  a  position  they  are  likely  to  become  damaged:  and  I  have 
already  explained  how  necessary  it  is  to  protect  the  c)Utsi('e  fibers 
of  the  wood.  Remember  also  that  wood  easily  }>ecome8  distorted. 
This  particularly  applies  to  the  interplane  struts.     The  best  method 


40  AIR   SERVICE   HANDBOOK. 

of  storing  struts  is  to  stand  them  uy)  in  as  near  a  vertical  position  as 
possible. 

6.  When  lifting  an  airplane  as  might  have  to  be  done  when  the 
landing  gear  is  In'oken,  it  is  convenient  to  lift  the  machine  by  putting 
one's  shoulders  under  the  main  spar  and  under  an  interplane  strut 
and  lifting  with  one's  back.  When  lifting  the  tail  of  a  machine 
lift  under  one  of  the  fuselage  struts  just  in  front  of  the  tail  plane. 
The  best  place  is  usually  marked  by  an  arrow. 

7.  Planes  kept  temporarily  in  the  hangars  should  be  kept  slung 
in  broad  bands  of  webbing.  A  2-inch  batten  should  be  threaded 
through  the  loops  of  webljing  and  the  leading  edge  of  the  plane  should 
be  placed  on  this  to  prevent  its  being  distorted. 

8.  Planes  packed  in  the  wing  trailer  should  he  supported  under 
the  compression  ri))s  on  pieces  of  felt.  These  pieces  of  felt  are  made 
to  fit  the  caml)er  of  the  plane  on  both  top  and  l)ottom. 

9.  When  a  ma'^'hine  is  standing  in  the  sheds  the  weight  of  the 
machine  should  be  taken  off  the  shock  absorbers.  This  is  con- 
veniently done  by  placing  the  landing  gear  on  lilocks  of  wood. 
The  tail  should  he  supported  on  a  trestle  in  a  proximately  the  flying 
position.  This  takes  the  weight  off  he  tail  and  prevents  the  fuselage 
being  in  a  continual  state  of  stress. 

KEEPING    .\X    .\IRPLANE    IN    GOOD       ONDITION. 

Cleanliness. — The  fal)ric  must  be  kept  clean  and  free  from  oil, 
otherwise  it  will  rot.  To  take  out  dirt  or  oily  patches  try  acetone. 
If  that  will  not  do  try  gasoline.  Both^acetone  and  gasoline  should 
be  used  with  caution  as  they  both  have  an  effect  on  the  dope  and 
varnish.  The  best  way  to  keep  the  planes  clean  is  to  use  soap  and 
warm  water,  but  in  that  case  be  sure  to  use  a  soap  having  no  alkali 
in  it  as  otherwise  it  will  badly  affect  the  faln-ic.  Use  water  sparingly 
or  it  may  get  insi<le  the  planes  where  it  tends  to  rust  the  wires  and 
swell  the  wooden  framework.  The  wheels  of  the  landing  gear  have 
a  way  of  throwing  up  a  great  deal  of  mud  on  the  lower  planes.  This 
should  be  taken  off  at  once.  Do  not  allow  it  to  dry  and  do  not  try 
to  scrape  it  off  when  dry.  If  it  is  dry  then  it  must  be  moistened 
first  as  otherwise  the  fabric  will  be  spoiled.  A  good  cleaning  solu- 
tion is  a  pail  of  warm  water,  with  soft  carriage  soap  and  some  Gold 
Dust. 

ControUlng  wires. — After  every  flight  pass  your  hand  over  the  wires 
and  carefully  examine  them  near  the  pulleys.  If  only  one  strand 
is  broken  the  wire  must  he  changed.  Not  only  does  a  ))roken  strand 
weaken  the  cable  hut  it  may  become  jammed  and  prevent  the  pilot 
using  one  of  his  controls.     Do  not  forget  the  aileron  balance  wire 


AIR  SERVICE  HANDBOOK.  41 

on  the  top  plane.  Once  a  day  try  the  tension  ol  the  control  wires 
by  smartly  movin<j  the  control  levers  about  as  explained  before. 

Wires. — See  that  all  wires  are  kept  well  t^'reased  or  oiled  and  that 
they  are  in  the  same  tension.  When  examining  your  wires  be  sure 
to  have  the  machine  on  level  ujound ,  as  otherwise  it  may  i^ei  twisted, 
throwing  some  wires  into  undue  tension  and  slackening  others. 
The  best  way,  if  you  have  time,  is  to,  pack  the  machine  up  into  its 
fljdng  position.  If  you  see  a  slack  wii'e  do  not  jump  to  the  conclu- 
sion that  it  must  be  tensioned.  Perhaps  its  opposite  wire  is  too 
tight,  in  which  case  slacken  it  and  possibly  you  will  find  that  will 
tighten  the  slack  wiie.  ("arefidly  examine  all  wires  and  their 
connections  near  the  propeller  and  l)e  sure  that  they  are  snaked 
around  with  safety  wire  so  that  the  latter  may  keep  them  out  of 
the  way  of  the  propeller  if  they  come  ailrift. 

Distortion. — Carefully  examine  all  surfaces  including  the  con- 
trolling surfaces  to  see  whether  any  distortion  has  occurred.  If 
distortion  can  be  corrected  by  the  adjustment  of  a  wire,  well  and 
good,  but  if  not  then  report  the  matter. 

Adjustment. — Verify  the  angle  of  incidence,  the  dihedral  angle, 
the  stagger,  and  the  overall  measurements  as  often  as  possiV)le. 
Alterations  in  all  these  cause  inefficiency,  but  as  one  measiu'ement 
usually  depends  on  another,  no  alteration  should  be  made  without 
orders  from  the  pilot. 

Landing  gear. — Constantly  examine  the  alignment  and  fittings  of 
the  landing  gear,  the  condition  of  tires,  shock  absorbers,  and  the 
tail  skid. 

Control  surfaces. — As  often  as  possible  verify  the  rigging  position 
of  the  ailerons  and  elevators.  This  should  be  done  by  the  pilot 
when  the  machine  is  in  flight  as  well  as  when  the  machine  is  on 
the  ground . 

Locking  arrangciuents. —  Constantly  inspect  the  locking  arrange- 
ments of  all  turnbuckles,  bolts,  etc. 

Outside  position. — The  aii'plane  when  outside  its  hangar  must 
always  stand  facing  the  wind.  If  this  is  not  so  then  the  wind  may 
catch  the  controlling  surfaces  and  move  them  sharply  enough  to 
<lamage  them.  If  the  airplane  must  be  moved  during  windy  weather 
then  the  control  levers  should  be  lashed  fast.  It  is  a  good  thing 
always  to  do  this  when  getting  out  of  the  machine,  but  the  lashing 
should  be  fixed  so  that  the  pilot  can  not  sit  down  and  therefore  can 
not  go  up  into  the  air  with  his  controls  lashed . 

Inspecting. — Learn  to  become  an  expert.  Whenever  you  have  the 
opportunity  practice  sighting  one  strut  against  another  to  see  that 
they  are  parallel.  Standing  in  front  of  the  maciiine.  which  in  such 
case  should  be  on  level  ground,  sight  the  center  section  plane  against 
the  tail  plane  and  see  that  the  latter  is  in  line.     Sight  the  leading 


42  AIR  SERVICE  HANDBOOK. 

edge  against  the  main  spars,  the  rear  spars,  and  trailing  edges,  taking 
into  consideration  wash  in  and  wash  out.  You  will  be  able  to  see 
the  shadow  of  the  spars  through  the  fabric.  By  practicing  this  sort 
of  thing  you  will  after  a  time  become  quite  an  expert  and  will  be  able 
to  diagnose  by  eye  false  efficiency,  stability,  and  control. 

Disnaantling  a  machine. — If  for  any  reason  the  machine  has  to  be 
dismantled,  the  engine  should  be  taken  out  first  of  all  and  then  the 
wheels  should  be  taken  off;  the  tanks  should  also  be  emptied.  This 
makes  the  machine  light  to  handle  and  will  save  it  from  damage  if  it 
falls  off  the  trestles,  which  are  often  very  inefficient  in  the  field.  If 
the  machine  will  have  to  be  rerigged  it  saves  a  lot  of  time  if  eAery 
cable  and  strut  is  labeled  and  if  all  turnbuckles  are  kept  on  their 
proper  cable.  Pins  should  be  put  back  in  their  proper  j^laces  and 
tied  in.  The  machine  should  be  kept  dismantled  as  short  a  time  as 
possible.  The  longer  a  machine  is  kept  so  the  more  the  jiarts  become 
lost  and  damaged. 

Packing  machine  in  a  truck. — Most  two-seater  machines  can  be 
packed  in  a  3-ton  truck.  After  the  tanks  have  been  emptied  the 
fuselage  can  be  placed  in  the  center  of  the  truck  and  supported  on 
suitable  j^acking  so  as  to  prevent  the  sockets  of  the  landing  gear  from 
being  damaged.  The  wings  can  be  placed  on  their  edges  on  either 
side  of  the  fuselage  and  it  is  convenient  to  prevent  them  from 
rubbing  by  supporting  them  on  grass  or  straw  and  putting  straw  ropes 
between  the  planes  themselves  and  the  truck.  The  control  surfaces 
can  be  tied  to  the  top  of  the  truck  and  they  should  be  slung.  If  they 
are  tied  with  string  passed  through  the  hinges  the  string  will  fray  on 
account  of  the  Adbration  of  the  truck. 

Piping  and  cable. — Piping  of  all  kinds  must  be  arranged  with  a 
proper  regard  to  the  amount  of  vibration  to  which  it  will  be  sub 
jected.  Long  unsupported  lengths  should  be  avoided.  Gasoline 
and  oil  pipes  usually  break  just  below  then-  attachments  to  the  tank, 
etc.  Vibration  is  more  evenly  distributed  by  fitting  pipes  with  a 
curl  K  just  below  the  point  of  support.  In  metal  piping  it  is  often 
advisable  to  ht  a  joint  of  specially  i)repared  rubber  tubing  close  to 
the  unions  to  prevent  fracture.  Although  special  rubber  tul)ing  is 
prepared  to  resist  the  action  of  gasoline  and  oil  it  will  nevertheless 
gradually  deteriorate  and  will  require  examination  and  renewal  at 
sMort  intervals.  Chokes  in  pipes  are  frequently  caused  by  deteriora- 
tion of  the  lining  of  the  tubing.  The  lining  is  often  damaged  by  the 
edge  of  the  tube  which  is  too  sharp. 

High-tension  cables  should  be  protected  at  the  points  where  they 
are  supported  to  prevent  the  covers  framing.  This  may  be  done  by 
binding  them  at  these  jioints  with  adhesive  tape. 


AIR  SERVICE  HANDBOOK.  43 

III.  SAIL  MAKING. 

TOOLS   AND   MATERIALS    USED    IN    SAIL   MAKING. 

1.  Tools: 

Scissors. 
Hammer. 
Ice  pick. 

Stringing  needles. 
Sewing  needles. 
Eyelet,  punch,  and  dies. 
Dope  and  varnish  brushes. 
Steel  tape  (50  feet). 
16K.33  .sewing  machino. 

2.  Materials: 

Fabric. 

Dope. 

Varnish  (light  span. 

Varnish  solvent  (V114  thinners). 

Acetone . 

Thread,  30,  40,  and  50. 

Hemp  string. 

Copper  tacks,  l-inch. 

Brass  brads,  A-inch . 

Eyelets,  bootblack. 

Protective  coloring  (colored  dope). 

Fabric. — The  fabrics  used  in  sail  making  are  the  best  Irish  linen 
and  cotton.  Its  weight  is  4  ounces  per  square  yard  and  this  is 
increased  to  tj  ounces  after  it  has  been  doped  and  varnished.  Before 
being  issued  for  use  all  fabric  is  subjected  to  the  light  test.  The 
method  of  this  test  is  to  pass  the  fabric  before  a  powerful  electric 
light  which  shows  very  plainly  all  the  flaws  in  the  weaving.  These 
flaws  are  marked  with  a  ])lue  pencil  and  when  covering  a  plane  all 
such  marks  are  carefully  covered  with  a  patch.  The  threads  which 
run  across  the  fabric  from  selvage  to  selvage  are  called  the  weft  and 
warp  is  the  lengthways  of  the  material.  Doped  fabric  is  stronger 
than  undoped  by  about  4  per  cent. 

Dope. — This  is  a  solution  of  cellulose  which  makes  fabric  air, 
gasoUne,  and  water  tight,  reducing  skin  friction  and  strengthening 
the  fabric.  Also  used  as  a  method  of  sticking  on  patches.  It  can 
also  be  used  on  wood  to  make  it  waterproof.  No  nitrate  base  cel- 
lulose should  be  used,  as  it  is  highly  inflammable. 

Varnish. — The  varnish  used  on  a  plane  is  special  light  airplane 
varnish.     Certain  varnishes  must  be  used  with  certain  dopes  and 


44  AIR  SERVICE  HANDBOOK. 

care  should  be  taken  that  the  proper  varnish  is  used.  Varnish 
increases  the  efficiency  of  dope,  i.  e.,  reduces  the  skin  friction  and 
the  ill  effects  of  differences  of  temperature,  wet,  etc. 

Solvent. — The  solvent  is  used  to  remove  the  varnish  from  a  plane 
without  unjuring  the  doped  surface  of  the  fabric.  It  can  be  used 
also  for  thinning  out  the  varnish. 

Acetone. — Can  be  used  for  thinning  dope.  It  should  be  used  with 
caution  for  cleaning  the  oil  and  dirt  off  the  plane.  Acetone  alone 
should  not  be  used  on  fabric,  as  it  tends  to  make  the  fibers  brittle. 

Protective  coloring. — When  varnish  is  used  on  the  top  plane  a 
colored  pigment  should  be  mixed  with  it.  This  mixture  is  called 
the  protective  coloring.  Its  vise  is  to  prevent  the  sun's  rays  rotting 
the  fabric  through  decomposition  of  the  dope.  The  coloring  should 
not  be  applied  to  a  varnished  plane  without  first  removing  all  varnish 
with  a  suitable  solvent. 

Covering. — Care  should  be  taken  before  commencing  to  cover  a 
surface,  and  the  same  applies  to  patching,  that  the  supporting 
trestles  are  placed  under  compression  ribs  or  main  spars.  Take 
note  that  all  turnbuckles  are  locked  and  greased  and  then  bound 
with  strips  of  fabric  or  adhesive  tape.  All  edges  of  ribs  and  main 
spars  are  then  covered  with  strips  of  fabrics  to  prevent  the  cover 
rubbing  on  the  wood.  Measure  width  of  plane  from  leading  to 
trailing  edge,  double  result  and  add  4  inches.  This  gives  the 
measure  or  length  each  section  of  the  cover  should  be  cut.  The 
fabric  varies  in  width  from  36  to  38  inches,  so  by  measuring  length 
of  plane  the  number  of  sections  or  pieces  of  fabric  required  for 
cover  can  be  found.  These  sections  are  then  machined  together 
by  means  of  a  seam  known  as  the  " balloon  seam . "  This  seam  is 
made  by  turning  the  selvage  of  the  fabric  in  a  half  inch  and  locking 
the  two  sections  together.     It  is  then  machined  on  either  side,  thus 


Fig.  19. 

This  seam  has  the  same  finish  on  each  side.  When  all  sections  re- 
quired are  seamed  together,  fold  the  cover  in  two  lengthways  with 
the  right  side  inside,  thus  finding  the  center.  Crease  the  double 
edge  with  the  fingers  and  lay  the  cover  on  the  top  side  of  the  leading 
edge,  keeping  the  creased  edge  of  the  cover  on  the  leading  edge. 
Tack  at  every  rib.  Eyelet  holes  are  placed  between  each  ril)  on  the 
underside  of  plane  at  the  trailing  edge.  This  is  done  before  over- 
sewing round  edge.  These  holes  are  for  the  escape  of  moisture  which 
collets  in  all  planes.     Strain  cover  over  to  trailing  edge  and  again 


AIR   SERVICE  HANDBOOK.  46 

tack  ou  each  rib.  Turn  plane  over  and  strain  and  tark  fabric  as  on 
top.  The  fabric  is  then  turned  in  on  top  and  undersides  at  trailing 
edge  and  drawn  together  by  means  of  a  stitch  known  as  the  "over- 
sewing stitch."  The  plane  is  then  ready  for  stringing.  An  up- 
holsterer's needle  and  hemp  string  is  used  and  a  stitch  is  placed 
every  3  inches  across  each  rib.  Place  the  needle  through  the  plane 
on  right  of  ril)  and  bring  down  on  left,  keeping  the  needle  close  to 
the  rib.     Knot  string  on  top. 

Doping  plane. — When  the  sewing  has  been  finished  the  plane  is 
given  a  coat  of  dope  and  this  coat  must  be  rubbed  well  in.  When 
(juite  dry  a  "i-iu'h  strip  of  fabric  is  placed  over  each  rib  and  doped  on. 
These  strips  are  frayed  out  at  sides  half  an  inch.  This  is  to  make 
them  adhere  to  the  cover  better.  All  edges  are  then  Ijound  with 
these  strips.  Another  coat  of  dope  is  then  rubbed  in  and  then  three 
more  coats  are  evenly  laid  on.  Each  coat  must  be  thoroughly  dry 
l>efore  the  nevt  is  applied  and  before  applying  each  coat  the  plane 
should  be  rubbed  down  well  with  a  piece  of  fabric.  This  is  to 
remove  all  roughness  and  imparts  a  good  polish  to  the  plane.  On  no 
a'.'count  use  anything  rough  in  the  nature  of  sandpaper  and  do  not 
apply  too  much  pressure  so  as  to  make  the  fabric  slacken  off.  Two 
(■oats  of  varnish  evenly  applied  completes  the  plane.  Doping  and 
varnishing  should  be  done  on  a  dry  day  and  in  a  hot  room,  tem- 
perature about  70°.  If  the  room  is  too  cold  the  varnish  will  dry 
with  all  the  brush  marks  showing,  but  if  the  room  is  kept  warm  the 
varnish  dries  much  slower  and  the  brush  marks  have  time  to  even 
out.  Dope  and  varnish  should  be  applied  with  a  flat,  4  inch  camel 's- 
hair  brush.  If  the  room  is  damp  or  if  the  fabric  is  damp  the  dope 
dries  in  white  patches  instead  of  without  color,  as  it  ought  to  do. 
In  some  planes  the  fabric  is  adjusted  on  the  bias;  that  is,  the  seams 
run  diagonally  across  the  plane  instead  of  straight.  This  is  supposed 
to  l)e  stronger  and  to  ])revent  the  fabric  tearing  in  case  it  were 
damaged. 

Redoping. — It  is  sometimes  necessary  to  redope  a  plane  or  part  of  a 
plane.  First  of  all,  take  off  all  the  varnish  with  a  suitable  solvent. 
This  is  quite  easily  done  and  does  not  take  long.  Rub  into  the  old 
dope  fresh  doj)e  which  has  been  thinned  with  acetone.  This  should 
l)e  rubbed  well  in,  so  as  to  soften  the  old  dope.  Do  not  attemj)t  to 
remove  the  old  dope  wdth  acetone,  because  the  acetone  will  probably 
dry  out  before  the  dope  is  properly  softened,  and  acetone  tends  to 
make  the  fibers  of  the  linen  brittle.  After  the  old  dope  has  been 
softened  by  two  coats  of  the  thin  dope  two  coats  of  ordinary  dope 
should  be  applied  and  then  two  coats  of  varnish,  as  is  done  when 
doping  the  plane  for  the  first  time. 


46  AIR  SERVICE  HANDBOOK. 


If  a  plaue  is  damaged  in  any  way  care  should  be  taken  to  insure 
that  none  of  the  internal  parts  are  damaged.  It  is  sometimes  neces- 
sary to  make  a  comparatively  large  cut  in  a  plane  to  insure  this, 
although  the  original  damage  is  slight.  If  a  cut  has  to  be  made  it 
should  be  made  in  a  fore  and  aft  direction.  In  repairing  or  patching 
a  wing,  first  examine  the  tear  and  judge  for  yourself  whether  it 
should  be  sewn  up  or  whether  the  damaged  portion  should  be  cut 
out,  taking  into  account  the  condition  of  the  fabric,  the  edges  of  the 
tear,  and  the  part  of  the  wing  affected.  If  the  damage  be  a  clean 
cut  running  straight  with  the  thread  of  the  fabric  the  edges  are  drawn 
together  with  the  herringbone  stitch.  The  varnish  is  now  carefully 
removed  with  a  suitable  solvent  and  the  hole  is  covered  with  a  patch 
square  or  rectangular,  three-quarters  of  an  inch  larger  than  the  tear. 
The  edges  of  the  patch  are  frayed  out  to  make  it  stick  better  and  the 
patch  is  stuck  on  with  dope.  \'arnish  on  a  plane  prevents  the  dope 
sticking.  A  second  patch  is  then  doped  over  the  first.  This  also 
should  be  frayed,  the  fraying  being  a  quarter  of  an  inch  deep. 
Fraying  allows  the  dope  to  get  a  firm  grip  of  the  edges  of  the  fabric 
and  makes  it  a  better  stream  line;  a  small  gain,  but  in  the  aggregate 
worth  considering.  The  threads  of  the  fraying  must  run  all  parallel 
to  each  other  and  must  be  fiat  with  the  first  coat  of  dope.  All 
patches  should  have  three  coats  of  dope  and  two  of  varnish,  these 
being  applied  as  described  above.  For  sewing  in  a  patch  cut  the 
damaged  portion  out  in  the  form  of  a  rectangle  and  slit  the  corners 
the  length  of  an  inch.  Turn  in  the  fabric  as  this  gives  a  firm  edge 
to  which  to  attach  the  patch.  Cut  the  patch  the  size  of  this  opening 
and  allow  half  an  inch  for  turning  in  the  edges.  Make  it  fit  the  hole 
tightly  and  then  sew  it  in  with  the  herringbone  stitch.  The  sewing 
must  be  perfectly  regular,  taking  care  that  the  stitches  are  all  the 
same  length  and  distance  apart,  as  the  strength  of  the  patch  depends 
on  the  regularity  of  your  sewing.  The  patches  are  put  on  tight  to 
pull  with  and  match  the  cover  of  the  wings,  which  is  well  strained 
on  in  the  first  place.  The  corners  are  often  sewn  very  badly. 
When  you  get  within  a  good  length  stitch  of  the  corner  the  outside 
stitches  must  wheel  round  regularly,  while  those  on  the  inside  are 
much  closer  together,  or  so  to  say,  marking  time.  Remember  that 
the  corner  of  a  patch  is  always  the  weakest  part.  Nothing  is  gained 
by  making  the  patch  circular,  because  the  edges  must  be  turned  in, 
and  this  in  itself  forms  a  corner.  Dope  this  patch,  put  on  a  second 
three-quarters  of  an  inch  outside  all  stitching  and  then  another  tliree- 
quarters  of  an  inch  larger  than  this.  Fray  patches  and  dope  on  as 
described  above.     If  you  have  a  small  patch  to  go  on  with  a  few 


AIR  SERVICE  HANDBOOK.  47 

stitclies  underneath  it  may  l>e  put  on  with  varnish  insteail  oi  dope, 
l)ut  this  does  not  apply  to  patches  which  have  to  be  sewn  in. 

Storing  fabric. — It  is  essential  that  fabric  should  be  kept  quite 
dry  and  clean,  and  it  should  therefore  be  stored  in  a  dry  place.  If 
moisttire  is  present  when  the  fabric  is  being  doped  the  dope  will 
not  ])enetrate  })roperly  and  turns  white.  Planes  when  covered 
should  be  left  in  the  doping  room  some  time  before  the  dope  L's 
applied  so  as  U^  attain  the  temperature  of  the  room. 


Doping  room.—  In  order  to  insure  good  results  in  doping  it  is  of 
vital  importance  that  a  special  doping  room  be  provided  and  great 
attention  [)aid  to  tlie  maintenance  of  a  uniform  temperature,  ^'aporp 
from  dope  are  heavier  than  air,  so  that  outlets  should  be  provided 
near  the  floor  to  extract  the  bad  air  from  the  room.  Fresh  air  out- 
lets should  be  provided  high  up  in  the  wall  opposite.  Incoming 
air  should  pass  over  hot  pipes  so  that  the  room  may  be  maintained 
at  a  uniform  temperature  of  65°  to  70*^.  At  the  front  the  nearest 
approach  to  this  will  l)e  a  tent  or  hangar  heated  tolerably  by  a 
brazier  so  that  doping  should  be  done  on  a  sunny  day  when  the  sun 
has  had  time  to  dry  up  the  atmosphere. 

Storing  of  dope. — I)o])e  is  affected  by  the  ultra-violet  rays  of  the 
sun.  so  that  it  should  be  kept  in  light-tight  vessels.  Dope  should 
not  be  stored  for  more  than  three  months  and  should  be  used  as  sup- 
plied. Special  solvents  are  used  for  loosening  stuck  stoppers  and 
for  washing  brushes.  Brushes  may  be  kept  immersed  in  dope  and 
the  whole  covered  by  an  inverted  jar  stuck  to  the  table  with  dope, 
thus  making  an  air-tight  joint.  Do])e  should  be  stored  in  a  room  at 
a  temperature  of  about  60°. 

Mi'lhofI  of  apphjing  rlope. — The  brush  should  be  well  dipped  into 
the  dope,  but  care  must  be  taken  that  drops  of  dope  are  not  allowed 
to  fall  on  the  fabric  as  the  lirush  is  being  carried  to  the  point  of  work. 
The  dope  must  I)e  applied  to  the  fabric  with  a  smooth,  backward  and 
forward  motion  and  air  bubbles  must  not  be  formed.  The  first  coat 
must  be  rubbed  well  in,  particular  care  being  taken  on  the  parts 
covering  the  woodwork  in  order  to  make  the  fabric  adhere  to  the 
wood.  The  dope  must  penetrate  well  through  the  fabric  in  order  to 
"rivet"  the  coating  to  the  fabric  and  prevent  it  peeling.  In  cases 
of  doping  schemes  where  a  thin  first  coat  is  provided  it  will  not  be 
ne  -essary  to  rub  this  coat  in.  lie.'ause  this  may  cause  drops  to  form 
on  the  inner  surface  of  the  fabric.  After  the  final  coat  of  dope  has 
been  applied  the  plane  should  be  left  as  long  as  possible,  preferably 
about  12  hours  before  the  varnish  is  put  on.  In  some  dopes  the  last 
coat  contains  the  pigment,  so  that  only  one  coat  is  necessary  and  can 


48  AIR  SERVICE  HANDBOOK. 

be  put  on  at  once.  Only  upper  and  vertical  surfaces  are  covered 
with  pigment.  The  lower  surfaces  are  covered  with  transparent 
varnish. 

Identification  marks  should  be  painted  on  immediately  after  the 
last  coat  of  dope  and  are  covered  with  the  last  coat  of  transparent 
varnish. 

Defects. — White  patches  are  due  to  moisture  in  the  air  or  fal^ric  or 
to  the  doping  being  done  at  too  low  a  temperature.  With  dopes  in 
which  acetone  substitutes  are  used  white  patches  frequently  occur 
in  the  first  ct)at,  but  should  disappear  when  a  second  is  applied. 

Blisters  are  due  to  doping  being  done  at  too  high  a  temperature. 
Patches  refusing  to  dry  with  formation  of  blisters  is  due  to  faulty 
dressing  of  the  faljric,  proba])ly  traces  bf  soap. 

•  Cracks  appearing  in  circles  may  be  spots  of  new  mold  in  the 
fabric.  Circular  cracks  also  appear  if  two  coats  of  copal  varnish  are 
applied.  Sunlight  on  unprotcted  dope  causes  it  to  deteriorate  and 
crack . 

Yellow  patches  appearing  some  time  after  the  doping  are  probably 
due  to  dressing  left  in  the  faljric. 

Sagginess  is  due  to  moisture  or  sometimes  to  the  doping  l^eing  done 
at  too  low  a  temperature. 

MARKINGS    ON    AN    AIRPLANE. 

All  airplanes  should  be  marked  as  follows: 

One  insignia  should  be  placed  on  each  end  of  the  upper  surface 
of  the  top  planes  and  one  on  the  under  surface  of  the  lower  planes. 
The  circumference  of  the  circumscribed  circle  should  just  miss  the 
wing  flaps. 

The  insignia  consists  of  a  five  pointed  star  colored  white  with  a 
blue  circumscribed  field;  the  center  of  the  star  is  a  red  circle;  the 
diameter  of  the  circumscribed  circle  will  he  equal  to  the  chord  of 
the  wing  on  which  the  insignia  is  placed.  The  diameter  of  inner 
circle  will  not  extend  to  the  inner  points  of  the  star  l)y  an  amount 
equal  to  one  twenty-fourth  of  the  diameter  of  the  circumscribed 
circle. 

The  inner  circle  should  be  painted  red  and  that  portion  of  the 
star  not  covered  by  the  inner  circle  will  be  painted  white;  the  re- 
mainder of  the  circumscribed  circle  should  be  painted  blue. 

The  rudder  should  be  painted  in  blue,  white,  and  red  vertical 
strips,  the  blue  strip  being  nearest  the  rudderpost. 

The  rudder  should  be  marked  with  the  machine's  number  in  3- 
inch  letters  on  the  top  of  the  white  baud. 

The  sides  of  body  of  the  airplane  should  be  left  free  of  all  markings 
except  such  as  shall  be  ordered  to  l)e  carried  in  the  field. 

One  point  of  the  star  in  each  insignia  will  point  to  the  front. 


AIB,  SERVICE  HANDBOOK.  49 

IV.  ENGINES,  MATERIAL. 

Steel. — Steel  is  a  form  of  iron  containing  a  certain  percentage  of 
carbon  and  in  some  cases  alloyed  with  small  quantities  of  other 
metals  such  as  nickel,  chromium,  vanadium,  or  manganese.  The 
amount  of  carbon  present  and  the  treatment  to  which  the  eteel  has 
been  subjected  determine  its  mechanical  properties. 

The  metal  iron  in  a  chemically  pure  state  is  only  found  in  a 
chemical  laboratory,  but  a  good  commercial  wrought  iron  is  reason- 
ably pure.  Wrought  iron  contains  a  very  low  percentage  of  carbon 
(up  to  about  0.25  per  cent).  It  is  ductile  (it  can  be  stretched), 
comparatively  soft  and  fibrous  in  structure.  Steel  contains  about 
0.25  (o  ab<nit  2  per  cent  of  carbon,  and  cast  iron  is  an  iron  containing 
from  about  2  to  5  per  cent  of  carbon.  In  most  cases  cast  iron  also 
contains  a  percentage  of  silicon. 

Steel  containing  a  low  percentage  of  carbon  is  called  mild  steel  and 
is  similar  to  wrought  iron  in  its  mechanical  properties.  It  is  com- 
paratively soft  and  ductile,  but  is  not  fibrous  in  structure.  It  is 
used  for  bolts  and  nuts,  operating-rod  brackets,  engine  bearers,  etc., 
and  in  general  engineering  for  steam  boilers,  bridge  gii'ders,  and  con- 
structional work  generally  where  a  brittle  metal  such  as  cast  iron  is 
unsuitable. 

As  the  percentage  of  carbon  is  increased  the  character  of  the  steel 
alters.  It  becomes  harder  and  more  brittle,  but  the  amount  of  hard- 
ness depends  upon  the  treatment  the  steel  has  received.  To  get 
maximum  hardness  a  steel  containing  a  fairly  high  percentage  of 
carbon  (e.  g.,  tool  steel  or  silver  steeD  should  be  heated  to  a  bright 
red  heat  and  immediately  quenched  in  cold  water  or  oil.  The 
effectiveness  of  this  hardening  process  depends  upon  the  rapid  cool- 
ing of  the  metal.  In  special  cases,  where  exceptional  hardness  is 
required,  the  steel  maj^  be  quenched  in  ice-cold  mercury,  which, 
being  a  good  conductor  of  heat,  brings  about  a  very  rapid  cooling  of 
the  metal.  A  high  carbon  steel  so  treated  is  called  "glass  hard." 
It  is  in  fact  hard  enough  to  cut  glass  and  may  be  used  for  this  pur- 
pose. It  is,  however,  brittle  and  unsuitable  for  most  purposes. 
Ordinary  files  and  hack-saw  blades  are  hardened  in  this  manner  and 
the  brittleness  of  these  tools  is  well  known  to  those  who  have  used 
them. 

Glass-hard  steel  is  too  brittle  for  ordinary  purposes  and  is  therefore 
softened  or  "let  down"  by  a  further  heat  treatment,  which  is  goner- 
ally  described  as  tempering.  In  this  process  the  steel  is  first  care- 
fully cleaned  so  as  to  prevent  a  bright  surface  and  heat  is  giadually 
applied.  Shortly  after  the  application  of  heat  a  film  of  oxide  begins 
to  form  on  the  bright  surface  and  by  the  color  of  this  film  the  amount 

46643—18 4 


fid  AIR   SERVICE   HANDBOOK. 

of  softening  can  be  estimated .  \A'hen  the  film  first  appears  it  is  a  very 
pale  yellow  to  an  orange,  orange  red,  purple,  and  deep  blue  to  a 
paler  blue,  after  which  the  metal  assumes  its  original  color,  and  if 
further  heat  is  applied  it  becomes  red  hot.  If  the  metal  is  quenched 
at  the  straw-color  stage  it  will  only  be  slightly  less  hard  than  it  waa 
before  "letting  down,"  and  if  it  is  quenched  at  the  pale-blue  stage  it 
will  only  l)e  slightly  harder  than  it  was  before  the  original  hardening. 
By  quenching  at  any  of  the  intermediate  points  a  corresponding; 
degi-ee  of  hardness  maybe  obtained.  The  (piality  possessed  by  a 
carbon  steel  of  responding  to  heat  treatment  as  outlined  above  is  erne 
of  the  most  important  properties  of  this  material. 

Mild  steel  may  be  hardened  on  the  surface  by  a  process  called 
"  casehardening, "  in  which  the  surface  of  the  metal  is  really  con- 
verted into  a  high  carbon  steel  and  then  quenched.  In  this  process 
the  mild  steel  articles  are  heated  to  a  red  heat  for  several  hours  while 
in  contact  with  a  substance  rich  in  carbon,  such  as  leather  charcoal, 
and  while  out  of  contact  with  the  air.  (  ams,  tappets,  steel  washers, 
and  the  races  and  balls  of  ball  bearings  are  treated  in  this  way,  the 
result  of  which  is  to  give  a  verj^  robust  structure  to  the  article  com- 
bined with  extremely  good  wearing  properties.  The  surface  is 
practically  glass  hard,  but  the  core  is  comparatively  soft  and  has 
all  the  toughness  of  hardened  steel.  (  ertain  steels,  notably  those 
composed  of  iron  carbon  and  tungsten  or  chromium,  are-  normally 
extremely  hard  and  retain  their  hardness  up  to  a  dull,  red  heat. 
These  are  used  for  high-speed,  heavy,  lathe  tools  and  for  the  exhaust 
manifolds  of  internal-combustion  engines,  where  they  frequently 
work  at  a  dull  red  heat.  Steels  composed  of  iron,  chromium,  and 
vanadium,  or  iron,  chromium,  and  nickel,  or  iron  and  nickel  are 
extremely  tough,  and  are  not  susceptible  to  fatigue.  Such  steels 
are  used  for  connecting  rods  and  crank  shaftSjin  internal-combustion 
engines.  Similar  steels  with  various  percentages  of  nickel, 
chromium,  etc.,  are  used  for  cylinders  of  rotary  engines,  gear-wheel 
cams,  inlet  valves,  etc.  While  the  steel  cylinders  of  several  water- 
cooled  stationary  cylinder  engines  are  machined  from  solid  carbon 
steel  (about  0.6  per  cent  carbon),  in  such  cases  the  valve  pockets 
are  made  separately  and  welded  onto  the  cylinder  by  means  of  the 
acetylene  flame,  the  water-jacket  of  mild-steel  sheet  being  welded 
on  afterwards. 

Another  important  (luality  of  steel  is  its  property  of  carrjdng  or 
retaining  magnetism.  The  magnets  of  the  magneto  are  usually 
referred  to  as  permanent  magnets;  that  is  to  say,  they  have  been 
magnetized  and  remain  magnetized.  Such  magnets  are  made  of  an 
iron,  carbon,  and  tungsten  steel  carefully  hardened  to  the  maximum 
extent.  Soft  steels  and  irons  may  be  easily  magnetized,  but  do  n(>i 
retain  their  magnetism.      Hard  ca.st   iron  retains  magnetism  fairly 


AIE   SERVICE   HANDBOOK.  91 

well,  l)ut  can  not  ho  inagnotized  to  the  samo  extent  as  steel.  The 
special  magnet  steels  for  permanent  magnets  iiermit  of  a  high  degree 
of  magnetism  and  retain  their  magnetism  to  a  remarkable  extent. 
The  armature  of  a  magneto  is  composed  of  a  xcry  soft  steel  which  is 
easily  magnetized  and  demagnetized. 

(JoKt  iro7i. — The  relatively  large  amount  of  carbon  ci)ntained  n 
cast  iron  (2  to  5  per  cent)  may  be  divided  into  two  parts:  that  con- 
tained in  the  iron  as  a  mechanical  mixture  in  the  form  of  graphite 
and  tliat  chemically  combined  with  the  iron.  The  Indk  of  the 
carbon  present  is  in  the  form  of  graphite  held  in  the  j)()res  of  the 
iron.  In  structure,  therefore,  cast  iron  resenil)les  a  sponge  of  iron 
with  the  interstices  tilled  with  grai)hite.  The  })resence  of  grai»lute 
obviously  weakens  the  metal  but  it  confers  a  special  ])roperty 
which  is  extremely  useful.  (Iraphite  is  a  lubricant  so  that  cast  iron 
may  be  looked  upon,  to  a  certain  extent,  as  a  self-Iuliricating  metal. 
In  ])ractice  it  is  found  that  cast  iron  surfaces  run  extremely  well 
together  in  machinery.  In  practically  all  engines,  with  the  exception 
of  aero  engines,  the  cylinders  and  piston  rings  are  made  of  cast  iron, 
and  in  internal-combustion  engines  it  is  usual  to  make  the  pistons 
also  of  cast  iron,  but  since  this  metal  is  relatively  weak,  cast-irou 
]nstons  and  cylinders  must  be  relatively  heavy  so  that  in  aero  engines 
the  pistons  and  fre(iuently  the  cj'linders  are  made  of  steel  or  alumi- 
nium alloy.  (  ast-irou  piston  rings  are,  however,  nearly  always  used. 
Where  steel  cylinders  are  in  use,  they  are  sometimes  fitted  with 
cast-iron  liners.  (  itst  iron  is  a  comparatively  brittle  metal,  about 
half  as  strong  as  steel.  It  can  not  be  hardened  and  tempered  as  in 
the  case  of  steel,  but  very  hard  chilled  castings  may  be  obtained 
by  using  a  special  kind  of  mold.  Soft  or  'malleable"  iron  castings 
are  ordinary  castings,  which  have  been  heated  for  a  considerable 
period  in  contact  \vith  an  iron  oxide  (red  hematite).  Cast  iron  is 
comparatively  strong  in  compression  and  weak  in  tension  so  that 
no  meml)er  of  an  engine  which  may  come  under  tension  is  composed 
of  cast  iron. 

Copper  is  a  soft  metal  of  extreme  ductility.  It  is  the  best  con- 
ductor of  electricity  with  the  exception  of  silver  only.  It  is  there- 
fore used  for  electrical  connections,  magneto  windings,  etc.  It  is 
easily  deposited  or  "plated"  in  an  electrolitic  bath,  and  in  some 
aero  engines  the  water  jackets  are  formed  of  cojjper  applied  in  this 
manner.  Copper  as  opposed  to  iron  becomes  brittle  after  being 
heated,  but  may  be  made  soft  again  by  being  '"worked." 

Brass,  an  alloy  of  copper  with  zinc,  usually  a])out  two  of  copper  to 
one  of  zinc  (by  weight).     It  is  very  easily  machined  and  casts  well. 


62  AIR  SERVICE   HANDBOOK. 

It  is  about  the  same  strength  as  cast  iron  and  can  be  made  hard  and 
springy  by  rolling.  It  is  used  for  obdurator  rings,  small  parts  of  car- 
bureters, magnetos,  etc. 

Bronze. — Bronzes  of  various  composition  are  used  as  bearing 
bushes,  tappet  guides,  small  gear  wheels,  etc.  Steel  runs  very  well 
on  bronze  and  the  wear  is  not  excessive.  Ordinary  bronzes  are 
alloys  of  copper  and  tin,  about  80  to  90  per  cent  of  copper  and  20  to 
10  per  cent  of  tin.  A  large  percentage  of  tin  giving  a  hard  and 
more  brittle  metal.  Ordinary  bronzes  also  are  not  much  stronger 
than  brass,  and  phosphor  bronze,  containing  about  2  to  4  per  cent 
of  phosphorus,  is  nearly  as  strong  as  steel.  Phosphor  bronze  is 
also  one  of  the  best  metals  for  bearing  bushes. 

White  metal. — Connecting  rods,  bearings,  and  crank-shaft  bearings 
are  lined  with  white  metal  (if  they  are  not  of  the  ball-bearing  type). 
White  metal  is  composed  of  tin,  copper,  and  antimony  in  varying 
percentages.  A  typical  case  is:  Tin,  90  per  cent;  antimony,  7  per 
cent;  and  copper,  3  per  cent.  This  gives  a  metal  of  low  melting 
point,  which  is  fairly  hard  and  runs  well  on  steel.  In  the  event 
of  a  bearing  tending  to  seize  up,  the  heat  generated  will  be  sufficient 
to  melt  the  white  metal,  and  if  the  engine  is  immediately  shut  down 
no  further  damage  will  result. 

Aluminum  and  aluminum,  alloys. — Aluminum  weighs  about  160 
pounds  per  cubic  foot  and  cast  iron  about  450  pounds  per  cubic  foot. 
Cast  iron  is  therefore  about  three  times  as  heavy  as  aluminum.  Pure 
aluminum  is  not  so  strong  as  cast  iron,  and  it  is  often  alloyed  with 
heavier  metals  with  the  object  of  increasing  its  strength.  Crank 
cases,  gear  boxes,  and  various  fittings  of  aero  engines  are  usually 
made  of  aluminum  alloy,  and  in  some  cases  the  pistons  are  made  of 
this  metal .  One  of  the  difficulties  in  the  use  of  aluminum  for  parts 
that  are  exposed  to  the  high  temperature  of  burning  gases  is  that  it 
has  a  comparatively  low  melting  point  (about  1,100°  F.)  and  becomes 
mechanically  weak  when  raised  to  a  high  temperature.  For  this 
reason  the  heads  of  aluminum  pistons  are  well  supported  by  means  of 
internal  webs,  and  in  some  cases  also  by  an  internal  pillar  resting 
on  the  gudgeon  pin  through  a  slot  in  the  top  of  the  connecting  rod's 
small  end.  These  webs,  etc.,  also  help  to  conduct  away  the  heat 
from  the  piston  head  and  so  further  reduce  the  risk  of  collapse. 
Another  difficulty  arises  from  the  fact  that  under  the  influence  of  heat 
aluminum  expands  at  nearly  twice  the  rate  of  iron,  thus  necessi- 
tating a  large  clearance  between  the  piston  and  the  cylinder.  It  is 
not  possible  to  solder  aluminum  in  a  satisfactory  manner.     Ordinary 


AIR  SERVICE  HANDBOOK.  68 

solder  is  quite  useless,  and  the  special  solder  sometimes  recommended 
requires  special  treatment  and  generally  gives  very  poor  results. 

Notes  on  distortion. — All  metals  expand  under  the  influence  of 
heat.  The  amount  of  expansion  is  proportionate  in  any  metal  to 
the  increase  in  temperature  but  differs  for  different  metals.  In  the 
case  of  aero  engines  the  Working  temperature  is  very  high,  owing  to 
the  high  compression  used,  the  high  speeds  at  which  these  engines 
run,  and  the  absence  of  large  masses  of  metal  which  would  help  to 
conduct  away  the  heat. 

If  the  piston,  cylinder,  and  Tahos,  and,  in  the  case  of  water-cooled 
engines,  the  Water  jackets.  Were  made  of  the  same  metal  or  of  metals 
expanding  at  the  same  rate,  and  if  they  Were  all  raised  to  the  same 
temperature  expansions  would  give  no  trouble.  In  practice,  how- 
ever, not  only  are  the  parts  made  of  different  metals  but  they  work 
at  dift'ering  temperatures  with  the  result  that  uneciual  expansion  and 
subsequent  distortion  takes  place. 

The  hottest  part  of  an  engine  is  the  exhaust  valve,  but  as  this  is 
a  small  symmetrical  part,  distortion  is  small.  The  exhaust-valve 
seating,  however,  will  probably  be  hotter  on  one  side  than  the  other, 
and  in  a  badly  designed  engine  the  distortion  will  be  so  great  as  to 
prevent  the  valve  seating  properly  wlien  the  engine  is  running  on 
low  throttle. 

The  inlet  valve  is  the  coolest  part  of  an  engine,  as  it  is  in  the  path 
of  the  cold  incoming  mixture,  and  in  some  types  of  engines  it  is 
placed  as  close  as  jjossible  to  the  exhaust  valve  with  a  view  to  keeping 
the  temperature  down.  In  some  engines  where  the  water  jacket 
is  of  mild  steel  or  copper  sheet  circumferential  ribs  or  corrugations 
are  made  in  the  jackets  in  order  that  they  may  more  easily  follow  the 
expansion  of  the  cylinders.  The  piston  head  becomes  very  hot,  as 
it  can  lose  heat  only  by  conduction  through  the  skirt  to  the  lower 
part  of  the  cylinder  wall  and  through  the  gudgeon  pin  to  the  con- 
necting rod.  The  net  result  of  distortion  is  that  in  practice  clear- 
ances have  to  be  made  larger  than  would  otherwise  be  necessary. 

Fatigue  of  metals. — Metals  Which  have  been  subjected  to  repeated 
stresses,  such  as  those  caused  by  vibration,  become  fatigued,  their 
internal  structure  changes,  and  they  are  permanently  Weakened. 
The  amount  of  fatigue  depends  upon  the  range  of  the  stress  or  load, 
the  number  of  times  the  material  is  subjected  to  the  stress,  and  the 
rate  at  which  the  stress  is  applied . 

The  prolonged  application  of  varying  stresses  very  much  smaller 
than  the  normal  breaking  stress  of  the  material  will  induce  fatigue 
and  eventually  bring  about  fracture. 


64  AIR  SERVICE  HANDBOOK. 

V.  THE    GASOLINE    MOTOR. 

Introductory. — The  object  of  a  motor  is  to  produce  rotary  motion 
either  in  itself  or  in  a  shaft.  To  get  this  motion  the  motor  must  be 
provided  with — 

(a)  A  piston  which  must  be  free  to  move  up  and  down  within  a 
cylinder. 

(6)  A  rod  attached  to  the  piston  termed  a  connecting  rod. 

(c)  Attached  to  the  other  end  of  the  connecting  rod  a  crank  shaft. 

(d)  Attached  to  the  crank  shaft  a  flywheel  or  its  equivalent. 

The  action  of  the  motor  is  similar  to  the  operation  performed  by 
a  man  turning  a  grindstone.  The  stone  corresponds  to  the  fiy wheel 
of  the  motor,  the  handle  to  the  crank  shaft,  the  man's  arm  to  the 
connecting  rod,  and  the  power  exerted  in  turning  the  stone  to  the 
exploded  charge. 

Power  can  not  be  produced  without  a  cause.  One  of  the  most 
effectual  methods  of  producing  power  is  the  expansion  of  gases.  If  a 
substance  such  as  gunpowder  is  exploded  in  a  cylinder  with  an  open 
end  (a  gun  for  example)  practically  the  whole  effect  of  the  explosion 
is  felt  at  the  muzzle;  and  if  a  bullet  is  placed  in  the  gun  in  front  of 
the  gunpowder  it  is  blown  out  wath  great  force.  This  is  exactly 
what  happens  in  the  gasoline  motor — a  mixture  of  gasoline  vapor  and 
air  is  ignited  within  the  closed  end  of  the  cylinder  and  the  force  of 
the  explosion  drives  the  piston  in  front  of  it. 

The  piston  in  moving  down  the  cylinder  carries  the  connecting 
rod  \^-ith  it  and  the  latter  in  its  turn  commimicates  its  motion  to  the 
crank  and  so  to  the  flywheel. 

The  flywheel  once  it  has  started  rotating  will  carry  on  its  motion 
for  an  appreciable  time  without  any  further  application  of  power. 
Consequently  it  will  communicate  its  motion  to  the  crank  and  so  to 
the  piston,  pushing  the  latt(?r  uj)  the  cylinder  again.  At  the  same 
time  by  forcing  the  piston  upward  the  burnt  gases  are  expelled  from 
the  cylinder  through  a  suitable  port  or  valve  and  by  an  arrangement 
to  be  described  later.  By  the  action  of  the  flywheel  the  piston  will 
again  descend,  traveling  along  the  same  path  as  it  did  when  the 
mixture  was  exploded,  but  this  time  the  piston  is  dragged  instead  of 
being  pushed. 

Immediately  the  dragguig  motion  begins  the  port  through  which 
the  burnt  or  exliaust  gases  escape  is  closed  and  a  similar  port  or  valve 
leading  to  the  mixture  and  inlet  pipe  is  opened .  The  downward  mo- 
tion of  the  piston,  produces  a  partial  vacuum  at  the  head  of  the  cyl- 
inder which  results  in  a  new  charge  of  explosive  mixture  rushing 
into  the  cylinder  through  the  port  which  has  just  been  opened. 

Just  after  the  piston  reaches  the  bottom  limit  of  its  stroke  this 
port  closes.  The  piston  is  then  pushed  up  the  cylinder  once  more 
and  the  mixture  is  coTnprcssfMl. 


AIK   SERVICE   HANDBOOK.  66 

It  may  here  be  noted  that  n-ithin  certain  limits  the  greater  the 
compretision  to  which  a  mixture  of  gaHoline  vapor  and  air  is  subject, 
the  ciincker  it  will  burn,  and  consecpiently  the  gi'eater  will  be  the 
force  of  the  explosion. 

When  compression  is  at  its  highest,  i.  e.,  when  the  piston  i.s  on  the 
point  of  reaching  the  top  of  its  stroke,  the  mixture  is  ignited  and  the 
explosion  occurs  forcing  the  j)iston  down. 

It  will  thus  be  seen  that  one  explosion  and  consequently  one  power 
stroke  occurs  every  two  revolutions  of  the  crank  or  four  strokes  of  the 
piston.  I'^or  this  reason  the  gasoline  motor  is  described  as  working 
on  the  four-cycle  principle. 

The  four-stroke  cycle  can  be  summarized  briefly  as  follows: 

(a)  The  suction  stroke:  The  piston  descends,  inlet  port  or  valve 
opens,  and  an  explosive  mixture  of  gasoline  vapor  and  air  is  sucked 
into  the  cylinder. 

(b)  The  compression  stroke:  Just  after  the  piston  has  reached  the 
bottom  of  the  suction  stroke  the  inlet  valve  closes,  piston  ascends 
and  compresses  the  mixture  'both  inlet  and  exhaust  valves  being 
closed). 

(c)  The  power  or  working  stroke:  Just  before  the  piston  reaches 
the  top  of  the  compression  stroke  the  explosion  occurs  and  the  piston 
is  forced  down  again. 

(d)  The  exhaust  stroke:  Just  before  the  bottom  of  the  power 
stroke  the  exliaust  valve  opens.  The  piston  ascends  and  the  burnt 
or  exhaust  gases  are  forced  out  of  the  cylinder. 

Suction  stroke. — The  intake  pipe  is  full  of  an  explosive  mixture 
of  gasoline  vapor  and  air.  The  intake  valve  is  open  just  after  the 
piston  starts  descending  in  the  cylinder.  That  is  when  the  crank 
is  about  5°  to  9°  past  top  dead  center.  The  piston  descending 
draws  this  explosive  mixture  into  the  cylinder.  As  it  is  descending 
very  fast  it  causes  a  partial  vacuum  in  the  cylinder  which  the 
incoming  gases  have  not  sufficient  time  to  fill  uj)  till  after  the 
piston  starts  ascending  in  the  cylinder.  So  the  inlet  valve  is  not 
closed  till  the  crank  has  rotated  to  about  18°  past  the  bottom  dead 
center.  In  some  fast-running  engines  this  angle  is  very  much 
bigger. 

The  compression  stroke. — As  soon  as  the  cylinder  is  as  full  of  the 
explosive  mixture  as  is  possible  and  when  the  inlet  valve  is  closed 
the  piston  still  ascending  the  cylinder  compresses  the  gases.  At  a 
variable  point,  normally  about  2()°  before  the  crank  reaches  the  top 
dead  center,  the  explosive  mixture  is  ignited.  The  mixture  takes  an 
appreciable  time  to  l)urn  and  it  is  ignited  so  that  when  it  is  com- 
pletely burnt  the  piston  has  finished  its  upward  travel  and  is  just 
starting  to  descend.  This  is  called  advancing  the  spark,  and  the 
amount  of  advance  depends  largely  on  the  speed  of  the  engine. 


66 


AIR  SERVICE  HANDBOOK. 


AIR  SERVICE  HANDBOOK. 


67 


The  power  stroke. — As  soon  as  the  piston  starts  descending  in  the 
cylinder  the  gases  begin  to  expand  and  push  the  piston  down  till 
the  crank  reaches  a  point  varying  between  45°  and  75°  from  the 
bottom  dead  center.  This  is  the  power  or  working  stroke.  The 
exhaust  valve  is  now  open  and  the  gases  rush  out  of  the  cylinder. 
This  early  opening  is  called  "giving  lead"  to  the  exhaust  valve, 
and  it  is  found  very  advantageous,  as  it  insures  an  effective  escape 
of  the  exhaust  gases  and  consequent  absence  of  pressure  against 


I  g  n  i  t  i  o  M 


eo+tom 

Dead 

centre 

Fig.  21. 

the  piston  on  its  return  stroke.     If  the  lead  given  to  the  exhaust 
valve  is  insufficient  the  engine  is  liable  to  overheat. 

The  exhaust  stroke. — The  exhaust  valve  remains  open  till  the 
piston  has  passed  to  the  bottom  of  the  cylinder,  ascended  to  the  top 
and  has  just  started  to  descend,  that  is  when  the  crank  has  gone 
about  1°  to  5°  past  the  top  dead  center.  The  valve  is  closed  when 
the  piston  is  just  past  the  top  so  as  to  insure  that  as  much  of  the 
burnt  gases  have  been  cleared  out  of  tlie  cylinder  as  possible.  There 
is  now  a  very  short  space  of  time  between  the  closing  of  the  exhaust 
valve  and  the  opening  of  the  inlet  valve.  This  is  to  make  certain 
that  the  explosive  mixture  on  entering  the  cylinder  will  not  come 
in  contact  with  the  hot,  burnt  gases  and  so  be  ignited  prematurely. 


58  AIR  SERVICE  HANDBOOK. 

DETAILED    DESCRIPTION    OF   THE    WORKING    OK  THE    GASOLINE    MOTOR. 

Arrangement  of  valves. — The  majority  of  motors  have  two  valve.'* 
or  ports  for  each  cylinder,  one  to  admit  the  explosive  mixture  and 
one  to  release  the  bmnt  gases  after  explosion.  The  former  is  termed 
the  inlet  and  the  latter  the  exhaust  valve  or  port.  The  most  common 
arrangement  for  aero  engines  is  that  in  which  the  seatings  for  the 
valves  are  placed  in  the  head  of  the  cylinder.  In  ordinary  motor-car 
engines  the  tops  of  the  cylinders  are  cast  with  small  extensions  to  one 
side,  and  in  each  of  these  extensions  is  the  circular  seating  on  which 
the  head  of  the  valve  rests.  The  valve  itself  consists  of  a  mushroom- 
shaped  head  with  a  long,  thin  stem,  the  whole  being  made  in  one 
piece. 

The  head  has  a  beveled  edge  which  fits  closely  onto  the  seating  of 
the  cylinder,  being  held  down  by  a  spring  mounted  on  the  stem. 
The  bottom  of  the  stem  when  the  valve  is  closed  and  the  engine  is 
warm  should  be  just  clear  of  what  is  termed  a  ""push  rod."  The 
push  rod  itself  is  raised  and  lowered  by  means  of  a  cam,  and  so 
communicates  its  motion  to  the  valve. 

From  the  description  of  the  cycle  of  operations  it  is  clear  that  each 
valve  must  open  and  close  once  in  every  two  revolutions  of  the 
crank.  It  will  therefore  be  seen  that  the  cams  operating  the  valves 
must  be  worked  at  half  the  speed  of  the  engine.  This  half-time 
speed  is  obtained  by  fixing  to  the  crank  shaft  a  gear  wheel  with,  say, 
16  teeth  and  providing  the  .shaft  carrying  the  cams  with  a  geai' 
wheel  having  32  teeth.  Then,  when  these  two  wheels  are  enmeshed 
and  the  engine  is  turning  the  cam  shaft  will  be  driven  at  half  the 
speed  of  the  crank  shaft.  Valves  worked  on  this  jjrinciple  are  called 
■'mechanically  operated  valves.''  Exhaust  valves  are  always 
mechanically  operated.  The  necessity  for  this  can  be  clearly  seen, 
because  at  the  moment  it  is  necessary  to  open  these  valves  they  are 
being  held  tight  .shut  by  the  pressure  of  the  gases  in  the  cylinder. 

Inlet  valves,  on  the  other  hand,  are  sometimes  automatically  oper- 
ated— that  is  to  say,  they  are  opened  by  the  suction  effect  caused  by 
the  piston  moving  down  the  cylinder,  the  exhaust  valve  of  course 
being  closed.  A  light  spring  is  fitted  to  the  valve  stem  to  bring  it 
back  onto  its  seating  at  the  end  of  the  suction  stroke.  The  auto- 
matic inlet  valve  is  not  as  a  rule  considered  advantageous  because  it 
is  extremely  hard  to  balance  all  the  springs  exactly  so  that  a  differ- 
ent amount  of  mixture  is  sucked  into  each  cylinder.  This  causes 
bad  running.  The  necessity  for  sucking  also  prevents  the  cylinder 
from  getting  as  much  of  the  mixture  as  it  would  if  the  valve  were 
opened  mechanically. 

Owing  to  the  very  high  pressure  generated  in  the  cylinder  during 
the  explosion  it  is  very  necessary  that  the  \alves  should  be  so 


AIR   SEKVICE  HANDBOOK.  69 

designed  that  the  pre.ssui*e  due  to  compreHsion  and  explosion  holds 
them  on  their  seatings  and  so  assists  them  to  become  gas-tight. 
For  this  reason  valves  are  always  designed  to  open  inward.  In 
some  cases  the  inlet  valve  is  placed  close  to  and  immediately  oppo- 
site the  exhaust  valve  so  that  the  inlet  gases  pass  over  the  exhaust 
valve  and  tend  to  keep  the  latter  cool. 

N'alves  are  "timed"  by  setting  them  to  oj)en  and  close  when  the 
l)iston  is  a  certain  distance  down  the  cylinder  or  when  the  crank  of 
the  engine  is  at  a  certain  angle.  All  valve  settings  must  be  taken 
with  the  engine  turning  in  the  ahead  direction,  so  as  to  avoid  any 
errors  due  to  play  in  the  various  gear  wheels,  etc.  If  the  engine  be 
turned  too  far  ahead  past  any  particular  setting,  turn  it  back  more 
than  the  amount  required  before  starting  to  take  the  readings  again. 

In  order  to  obtain  the  direction  of  the  revolution  of  an  engine,  turn 
it  by  hand.  The  inlet  valve  wdll  open  directly  after  the  exhaust 
valve  closes  if  the  engine  or  crank  shaft  is  being  turned  in  the  correct 
direction.  By  watching  the  inlet  valves  the  order  in  which  the 
cylinders  fire  can  be  determined. 

When  taking  down  an  engine  for  examination  and  repairs  it  is 
absolutely  necessary  to  note  most  carefully  the  relative  positions  of 
the  timing  gear  wheels.  They  should  be  marked  unmistakably 
(usually  done  by  the  makers)  so  that  they  can  be  put  back  in 
exactly  the  same  relative  positions  as  those  in  which  they  were 
found . 

It  has  been  said  that  the  valves  must  fit  very  accurately  onto  their 
seatings.  If  the  engine  overheats,  the  valves  are  liable  to  warp,  and 
this  will  prevent  them  fitting  securely.  In  a  well-designed  engine 
this  should  not  wcur. 

Sometimes  little  bits  of  carbon  lodge  between  the  valves  and  the 
valve  seats.  If  this  happens  the  hot  gases  rush  across  and  in  doing 
so  will  soon  wear  away  the  valve  and  the  seat  so  that  the  compression 
in  the  cylinder  becomes  very  poor. 

In  time  the  guides  for  the  valve  stems  become  worn  and  the  valve 
instead  of  closing  squarely  will  close  on  one  side  before  the  other. 
This  also  allows  ihe  hot  gases  to  rush  ])ast  the  valve  and  wear  it  away. 

From  time  to  time  the  valves  must  be  "ground  in."  This  is  done 
by  coating  the  bevel  of  the  valve  with  valve-grinding  compound, 
which  usually  consists  of  a  paste  made  of  fine  emery  powder.  The 
valve  is  now  pressed  onto  its  seat  and  turned  around  by  means  of  a 
screw  driver  or  special  tool.  It  should  be  turned  both  ways,  and 
after  every  turn  or  two  should  be  lifted  out  of  its  seat.  This  makes 
the  bevel  even,  so  that  the  valve  will  close  properly  when  turned  in 
any  position. 

The  valve,  when  properly  ground,  should  make  a  gasoline-tight 
joint  with  the  valve  seat  and  can  be  tested  for  leakage  with  gasoline. 


60  AIR  SERVICE  HANDBOOK. 

In  many  engines  the  push  rod  is  done  away  with  and  the  vahes 
are  operated  directly  from  the  cam  through  a  rocking  arm. 

The  cylinder. — The  cylinders  of  an  engine  are  usually  made  of  steel. 
In  some  engines  the  cylinders  are  lined  with  cast  iron  and  in  others 
the  cylinders  may  be  made  of  cast  iron  altogether.  The  valve  seats 
are  either  welded  to  the  head  or  screwed  in.  There  is  a  hole,  or 
sometimes  two,  in  the  head  of  the  cylinder  screw  threaded  to  fit 
the  spark  plug.  In  an  air-cooled  engine  the  cylinder  will  carry  fins 
on  the  outside  and  in  the  case  of  a  water-cooled  engine  the  water 
jacket  will  be  welded  to  the  cylinder.  In  some  engines  the  jacket  is 
made  of  copper  deposited  electrically. 

Near  the  bottom  of  the  cylinder  will  be  a  means  of  attaching  it  to 
the  crank  case. 

Most  of  the  wear  on  the  piston  will  come  on  one  side,  and  after  the 
engine  has  run  over  100  hours  this  may  become  large  enough  to  inter- 
fere with  the  efficiency  of  the  engine. 

The  piston. — The  piston  can  be  described  as  a  hollow  cylindrical 
plug,  to  the  interior  of  which  is  hinged  the  connecting  rod.  This  is 
done  by  means  of  a  short  circular  steel  bar  called  the  gudgeon 
pin,  which  is  set  diametrically  through  the  piston  and  secured  firmly 
to  it.  It  is  important  that  the  gudgeon  pin  be  held  firmly  in  the 
piston  and  also  in  the  lugs  which  hold  it  to  the  piston.  The  gudgeon 
pin  is  generally  known  as  the  wrist  pin. 

The  piston  is  made  of  slightly  smaller  diameter  than  the  cylinder 
(about  8/1,000  inch  for  a  4-inch  cylinder)  in  order  that  it  may  move 
freely  up  and  down  the  cylinder.  This  clearance  depends  on  the 
materials  of  which  these  two  parts  are  made.  On  account  of  this 
clearance  it  is  evident  that  if  other  arrangements  were  not  made  the 
gases  would  leak  past  the  piston,  resulting  in  considerable  loss  of 
compression.  This  difficulty  is  surmounted  by  cutting  one  or  more 
grooves  around  the  outside  of  the  piston  wall  into  which  "piston 
rings"  are  fitted.  These  rings  are  made  of  slightly  larger  diameter 
than  the  bore  of  the  cylinder  and  are  cut  through  sometimes  diagon- 
ally and  sometimes  in  the  form  of  a  step;  thus,  when  the  piston  is  in 
the  cylinder  the  rings  are  compressed.  At  the  same  time  they  are 
constantly  trying  to  expand  to  their  normal  diameter,  with  the  result 
that  they  press  tightly  against  the  cylinder  walls  and  keep  the 
piston  gas-tight,  ^^^len  two  or  more  rings  are  employed  the  slits  in 
the  rings  must  not  be  vertically  over  each  other.  They  must  be  set 
in  different  positions  round  the  piston  so  as  to  avoid  as  far  as  possible 
the  escape  of  any  gases  past  the  slits  as  would  occur  were  they  in 
line.  The  ends  of  these  rings  must  be  some  distance  apart  when  cold 
(about  3/100  inch  for  a  4-inch  piston)  so  as  to  allow  for  expansion 
when  the  rings  become  hot. 


AI&  SEILVICE  HANDBOOK. 


61 


The  pistou  is  made  wath  a  large  skirt  so  as  to  have  a  l^rge  bearing 
surface  on  the  cylinder  walls.  This  also  prevents  the  piston  from 
tilting  in  the  cylinder  and  helps  to  conduct  away  the  hoat  from  the 
piston  head. 

The  bottom  ring  on  the  pistou  does  not  help  much  to  seal  the 
escape  of  gas  but  it  wipes  the  excess  of  oil  from  the  cylinder  walls 
and  prevents  it  from  getting  into  the  combustion  chamber  where 
it  would  carbonize  and  soot  up  the  engine  The  top  piston  ring  in 
some  rotary  engines  is  made  of  L  section  brass  and  is  called  an 
obdurator  ring  and  acts  in  exactly  the  same  way  as  does  the  cup 
leather  in  a  pump. 

The  connectinfi  rod. — The  connecting  rod  is  the  bar  which  connects 
the  piston  to  the  crank  pin.  It  is  usually  made  of  H  section  steel. 
The  small  end  is  fitted  to  take  the  bronze  bearing  of  the  gudgeon  pin. 
The  big  end  is  fitted  to  take  the  big-end  bearing,  which  consists  of  a 
cylinder  of  brass  lined  with  white  metal.     In  some  engines  of  the 


n--.'- 


?r.^.)l. 


Fig.  22. 

V  type,  only  one  of  the  pair  of  connecting  rods  bears  on  the  crank 
pin.  The  other  bears  on  the  outside  of  the  brass  cylinder  which  holds 
the  white  metal.  The  two  connecting  rods  thus  work  on  the  game 
pin. 

In  some  rotary  engines  there  is  one  rod  called  the  master  rod,  and 
this  is  the  only  one  which  bears  on  the  crank  pin.  All  the  other 
connecting  rods  are  hinged  to  flanges  on  the  master  rod  by  means  of 
wrist  pins. 

The  rod  being  of  H  section  must  not  be  bent  or  twisted,  as  this  will 
destroy  its  strength  altogether. 

The  crank  shaft. — The  crank  shaft,  usually  a  steel  forging, 
revolves  in  the  bearings  in  the  crank  case.  In  multicylinder 
engines  there  is  usually  a  bearing  between  every  two  crank  pins. 
These  bearings  may  be  ball  bearings  or  made  of  white  metal.  If 
the  propeller  is  carried  on  one  end  of  the  crank  shaft  the  shaft  carries 
a  thrust  bearing,  and  this  bearing  is  usually  made  to  take  the  thrust 
in  both  directions,  so  that  the  engine  may  be  used  in  a  pusher  or 
tractor  machine. 

The  crank  case. — The  crank  case,  made  of  aluminum  or,  in  the  case 
of  rotary  engines,  of  steel,  carries  the  cylinders  and  bearings  for  the 


62 


AIR  SEEVICE  HANDBOOK. 


crank  shaft  aud  also  carries  the  means  of  attaching  the  engine  to 
the  machine.  The  bottom  of  the  crank  case,  except  in  rotary 
engines,  is  usually  little  more  than  a  cover.  It  catches  the  surplus 
oil  and  sometimes  carries  the  pumps  which  return  this  oil  to  the 
main  oil  pump.  It  is  constructed  of  very  thin  material  and  (>ngines 
must  never  be  allowed  to  rest  "with  their  weight  on  the  crank  case. 
The  cam  shaft. — The  cam  shaft  carries  the  cams  which  operate  the 
valves  and  as  the  latter  are  machined  and  solid  with  the  shaft  it  is 
only  necessary  to  time  one  cam  and  the  rest  will  be  automatically 
adjusted.  In  some  engines  the  cam  shaft  carries  the  propeller,  in 
which  case  it  is  furnished  with  the  thrust  bearing. 


Ta1«.J.\' 


' 

0..-.  e  ,c  ro.  a.^ 

t   ^^.~.    »_r 

J 

LjiL-CX^..,t,<r 

-Ar».^^ 

4Cr 

l"'»j 

Fig.  23. 

Gears. — Since  the  cam  shaft  runs  at  half  the  speed  of  the  crank 
shaft  it  is  necessary  to  gear  them  together.  In  case  one  of  these 
wheels  has  to  be  replaced  it  should  be  remembered  that  the  type  of 
teeth  in  each  wheel  must  be  the  same.  If  the  teeth  "bottom" 
there  will  be  a  tremendous  vibration  in  the  engine.  If  there  is  too 
much  play  between  the  teeth  there  will  again  be  vibration  with 
every  little  change  of  speed  in  the  engine. 

The  carbureter. — This  term  is  applied  to  the  apparatus  which  is 
responsible  for  the  regular  supply  of  explosive  mixture  to  the  cylin- 
ders. One  of  the  most  important  factors  in  the  efficient  running  of 
a  gasoline  motor  is  the  mixture.  It  is  essential  that  the  particles  of 
gasoline  vapor  and  air  should  be  mixed  as  intimately  as  possible 
before  they  reach  the  cylinder.     This  is  what  the  carbureter  does. 


AIR   SERVICE   HANDBOOK.  63 

The  ga.solinc  is  led  iroiii  the  tank  in  the  machine  into  what  it* 
terniod  the  "float  chamber.  "  The  object  of  this  chamber  is  to 
keej)  the  liead  of  Liasoline  at  a  constant  h'vel.  Inaich'  the  chamber 
is  a  hollow,  brass  float.  Through  the  center  of  the  Hoat  a  needle 
))asses  which,  when  the  gasoline  has  risen  to  a  high  enough  level  in 
the  chamber,  (its  down  into  a  seat  in  the  gasoline  pipe,  thus  cutting 
off  a  further  supply.  Just  above  the  top  of  the  Hoat  two  balance 
weights  are  attached  to  the  needle.  The  weights  are  pivoted  about 
the  needle  and  rest  on  the  top  of  the  float.  Thus,  as  the  level 
rises  in  the  chamber  the  weights  are  pushed  up  and  eventually 
allow  the  needle  valve  to  fall  back  on  its  seat  and  so  stop  the  sup])ly 
of  gasoline. 

When  the  lexcl  of  the  gasoline  in  the  tloal  chamber  falls  the  float 
(Iroi)s  and  the  balanc<'  weights  acting  on  the  needle  valve  lift  it  and 
allow  a  fresh  supply  of  gasoline  to  come  from  the  tank.  Means  are 
provided  for  lifting  the  needle  off  its  seat  by  hand  with  a  view  to 
Hooding  the  carbureter  before  starting  the  engine. 

Carbureters  must  never  be  "  tickled, "  as  this  wears  the  needle  and 
makes  it  a  bad  fit  on  its  seat.  The  needle  should  simply  be  lifted 
until  the  carbureter  Hoods  and  then  dropped. 

.\  small  pipe  leads  the  gasoline  from  the  float  ciianiber  into  the 
jet  chamber.  Screwed  into  the  end  of  this  pipe  is  a  vertical  jet  or 
nozzle.  A  set  of  jets  with  different-sized  artifices  can  be  obtained 
for  use  under  varjdng  atmospheric  conditions.  The  top  of  the  jet 
is  arranged  at  such  a  height  that  it  is  very  nearly  the  same  height  as 
the  level  of  gasoline  in  the  carbureter  when  the  needle  valve  is 
closed  and  when  the  engine  is  in  a  normal  condition,  \^^len  the 
engine  is  running  a  partial  vacuum  due  to  the  suction  effect  of  the 
engine  occurs  round  the  jet  and  as  the  latter  is  small  the  gasoline  is 
emitted  in  a  fine  spray,  a  condition  which  makes  vaporization 
easy  and  consefjuently  admits  of  a  more  perfect  mixing  with  the 
air  being  sucked  past  the  jet  than  would  be  the  case  if  the  gasoline 
were  not  vaporized.  In  some  carbureters  the  vaporization  of  the 
gasoline  is  further  assisted  by  warming  the  supply  of  air  to  the 
carbureter  or  jacketing  the  inlet  pipe  with  hot  aii-  or  water. 

An  inverted  double  cone  is  sometimes  fitted  round  the  jet  to 
increase  the  speed  of  the  air  past  it  thereby  still  further  reducing 
the  pressure  at  this  point.  This  also  causes  the  difference  of  pres- 
sure on  the  gasoline  in  the  float  chamber  and  on  the  jet  orifice  to  be 
increased,  resulting  in  an  increased  flow  of  gasoline  without  inter- 
fering with  the  fineness  of  the  spray.  From  the  jet  chamber  the 
mixture  passes  along  the  induction  pipe  to  the  cylinder. 

The  action  of  the  float  can  be  likened  to  that  of  the  automatic 
water  system  with  its  ball  valve,  while  the  action  of  the  jet  can  be 
compared  to  that  of  a  perfume  spray. 


64  AIR  SERVICE  HANDBOOK. 

The  aboA'e  are  the  essentials  in  the  carbureter.  The  types  in 
general  use  are  more  complicated  and  arranged  for  the  suitable 
supply  of  gasoline  vapor  when  the  engine  is  running  in  dense  air 
near  the  ground  or  in  the  rarehed  air  at  a  height. 

The  throttle  and  air  valve. — The  intake  manifold  is  provided  with 
a  valve  called  the  throttle,  by  means  of  which  the  amount  of  mixture 
admitted  to  the  cylinders  can  he  regulated.  The  more  this  valve  is 
opened  the  greater  will  be  the  c^uantity  of  mixture  admitted  and  the 
faster  the  engine  will  run.  But  the  faster  the  engiae  runs  the  greater 
will  be  the  suction  effect  at  the  jet;  consequently  the  mixtiure  will 
become  richer  in  gasoline  unless  some  means  is  employed  for  admit- 
ting more  air.  This  is  usually  done  by  making  the  throttle  act  on 
both  the  inlet  and  discharge  side  of  the  jet  or  else  by  providing  an 
additional  air  port  the  size  of  which,  and  therefore  the  quantity  of 
air  admitted,  can  be  varied  at  will.  Another  method  is  to  pro\T.de 
the  extra  air  port  with  a  spring  which  allows  the  port  to  open  wider 
as  the  suction  in  the  intake  pipe  increases. 

The  intake  manifold  should  be  as  short  as  possible  and  should 
contain  no  sharp  curves.  It  should  be  large  enough  and  placed  in 
such  a  position  that  all  cylinders  get  the  proper  amount  of  mixture 
and  so  that  one  cylinder  does  not  starve  another. 

The  muffler. — At  the  end  of  the  working  stroke  there  is  always  a 
pressure  in  the  cylinder  above  that  of  the  atmosphere,  and  when  the 
exhaust  valve  opens  the  gases  rush  out  into  the  surrounding  air  at  a 
high  speed  and  with  much  noise.  To  reduce  the  noise  a  muffler  is 
usually  fitted,  consisting,  essentially,  of  a  large  vessel  into  which  the 
waste  gases  pass  direct  from  the  engine.  This  vessel  has  a  compara- 
tively small  exit  hole  for  the  gases  to  escape  through  to  the  atmos- 
phere. The  result  is  that  instead  of  rushing  into  the  air  with  a  series 
of  loud  reports  they  escape  in  a  steady  stream.  Baffle  plates  are 
often  fitted  in  the  muffler.  The  muffler  reduces  the  power  of  the 
engine  on  account  of  the  obstacles  the  gases  meet  on  their  way  to 
the  air.  Consequently  the  piston  has  to  do  more  work  in  forcing 
them  out  of  the  cylinder.  In  some  engines  it  is  possible,  however, 
to  fit  an  exhaust  pipe  in  such  a  manner  that  one  cylinder 's  exhaust 
assists  another  cylinder's  exhaust. 

Lubrication. — The  lubrication  of  bearings  is  carried  out  by  the 
formation  of  a  very  thin  film  of  oil  between  the  moving  surfaces, 
which  must  be  truly  aligned  and  worked  to  a  smooth  surface,  other- 
wise the  film  of  oil  will  be  broken  at  the  "hard  places,"  where  the 
metal  will  become  scored  and  the  bearing  probably  overheated. 

This  film  of  oil  is  formed  by  the  relative  motion  of  the  surfaces  and 
the  higher  relative  velocity  and  the  more  viscous  the  oil  the  more 
stable  will  the  film  become. 


AIR  SERVICE  HANDBOOK.  66 

At  low  speeds,  especially  under  heavy  loads,  the  oil  film  is  liable 
to  be  Bquashed  from  between  the  bearing  surfaces  and  the  lubrication 
will  then  largely  depend  on  the  ''greasiness"  of  the  surfaces.  For 
this  reason  slow-moving  toothed  gears  are  better  lubricated  by  a 
thick  grease  than  any  sort  of  oil. 

At  high  speeds  the  film  of  oil  will  form  between  the  moving  surfaces 
even  if  the  oil  is  faiily  thin,  but  if  the  load  is  gi-eat  the  lubricant 
must  be  of  a  more  greasy  nature. 

The  animal  and  vegetable  oils  (i.  e.,  castor,  sperm,  etc.")  are  more 
greasy  than  the  mineral,  and  so  must  be  used  under  hea\  y  loads, 
even  where  the  speed  is  high.  If,  however,  the  oil  is  forced  through 
the  bearings  under  pressure  and  is  retjuired  to  remain  in  contact 
with  the  working  parts  and  to  be  used  over  and  over  again,  mineral 
oils  must  be  emploj-ed.  Under  these  latter  conditions  vegetable 
and  animal  oils  become  acid  and  gummy  and  are  therefore  un- 
suitable. 

Water  is  a  very  bad  lubricant,  since  it  is  not  viscous  enough  to 
form  a  film  between  moving  surfaces,  nor  is  it  greasy,  and  so  great 
care  should  be  exercised  to  exclude  it  from  all  working  surfaces. 

The  oils  more  commonly  used  are  enumerated  below.  They  all 
weigh  rather  less  than  water.  The  vegetable  and  animal  oils  are 
liable  to  "gum"  by  oxidation,  biit  mineral  oil  is  free  from  this  de- 
fect and  can  be  used  again  and  again  provided  it  is  filtered  each 
time  before  reuse.  For  these  reasons  mineral  oil  is  employed  for 
lubrication  of  all  internal-combustion  engines  with  the  exception 
of  the  Gnome  and  one  or  two  other  types,  where  the  oil  simply 
passes  through  the  engine  and  then  escapes. 

Mineral  oil  (light)  for  forced  lubrication  of  bearings  and  low- 
powered  internal-combustion  engines  (water  cooled). 

Heavy,  filtered  mineral  oil  for  large  internal-combustion  engines 
(air  cooled). 

Mineral  grease,  vaseline,  for  preserving  machinery  and  also  the 
lubrication  of  the  gear  boxes,  etc. 

With  internal-combustion  engines,  since  the  oil  comes  into  con- 
tact with  very  hot  surfaces,  such  as  the  piston,  etc.,  an  oil  with  a 
flash  point  of  over  250°  F.  should  be  used.  A  fine  mineral  oil  which 
is  suitable  for  all  bearings  and  working  surfaces  is  generally  em- 
ployed. 

The  different  methods  employed  for  lubricating  gasoline  engines 
are  many,  but  they  can  be  classed  generally  under  two  heads: 

1.  Splash  lubrication:  In  this  method  the  engine  is  started  with 

oil  in  the  crank  case  up  to  a  certain  level.     Additional  supplies  of 

oil  are  pumped  into  the  crank  case  periodically  when  the  engine  ie 

running.     The  crank  throws  of  the  engine  revolve  into  the  oil  in  the 

46643—18 5 


66 


AIR  SERVICE  HANDBOOK. 


crank  case  and  splash  it  up  to  the  piston  and  cylinder  walls,  etc. 
A  baffle  plate  is  usually  fitted  at  the  bottom  of  the  cylinders,  leaving 
just  sufficient  room  for  the  travel  of  the  connecting  rod.  This  pre- 
vents overlubrication  (and  so  carbonization)  of  the  piston  and  cyl- 
inder walls  and  sooted  plugs. 

2.  Forced  lubrication:  In  the  forced  lubrication  system  a  certain 
amount  of  oil  is  delivered  by  means  of  a  pump  to  the  main  bearings, 
thence  by  means  of  a  hole  through  the  crank  shaft  to  the  main  bear- 
ings, and  then  by  a  pipe  or  grooves  along  the  connecting  rod  to  the 
wiist-pin  bearing.  A  separate  lead  also  supplies  the  cam  shaft  and 
healings  and  its  gear  wheels.  Oil  is  also  delivered  under  pressure 
to  the  valve-rocker  arms.  The  oil  streams  into  all  the  bearings  and 
keeps  them  well  lubricated,  so  that  if  well  fitted  there  is  extremely 


little  wear  in  any  of  the  bearings  thus  fed.  The  oil  when  it  has 
passed  through  the  bearings  falls  into  a  sump  in  the  lower  part  of 
the  crank  case,  whence  it  is  pumped  back  immediately  into  the 
tank  which  supplies  the  main  oil  pump.  No  oil  is  thus  allowed  to 
collect  in  the  crank  case,  so  that  the  engine  can  be  worked  in  any 
position  without  the  cylinders  at  one  end  becoming  overlubricated. 

The  oil  used  is  mineral.  As  it  is  circulated  round  the  system  and 
used  over  and  over  again  it  is  necessary  to  filter  it  between  each 
round  through  efficient  strainers  so  that  no  carbon  or  foreign  sub- 
stance is  forced  into  the  bearings.  Any  grit  (carbonized  oil,  etc.) 
in  the  lubricant  would  of  course  at  once  produce  local  heating  of  the 
bearing. 

A  pressure  gauge  is  provided  with  this  system  and  from  5  to  55 
pounds  pressure  per  square  inch  is  maintained  by  the  pump,  accord- 
ing to  the  type  of  engine.  Should  the  pressiure  fall  below  this  it  is 
usually  due  either  to  the  strainer  getting  choked  and  checking  the 


AIH  SERVICE  HAI7DB00K.  67 

supply  of  oil  to  the  pump  or  to  the  level  of  oil  falling  too  low  and 
causing  loss  of  suction. 

It  may  here  be  noticed  that  the  pressure  shown  when  the  engine 
is  first  started  will  be  considerably  above  that  which  may  be  expected 
after  the  engine  has  been  running  long  enough  to  heat  the  oil. 

Oil  pumps. — The  most  usual  oil  pumps  are  of  the  plunger  type  or 
the  gear  type.  They  are  worked  through  a  gearing  from  the  engine 
crank  shaft.  The  plunger  type  does  not  call  for  much  remarks. 
The  gear  works  as  follows: 

Two  gear  wheels  are  enmeshed  in  a  small  case  which  fits  closely 
around  them.  The  oil  is  supplied  to  one  side  of  these  wheels.  As 
they  rotate  they  carry  around  oil  in  the  spaces  between  the  teeth 
from  the  intake  side  to  the  output  side.  When  the  teeth  mesh  the 
oil  gets  squeezed  out  of  the  spaces;  it  can  not  go  back  past  the  wheels 
because  the  case  fits  tightly,  so  it  has  to  go  through  the  output  pipe. 
This  type  of  pump  gives  a  constant  flow  of  oil  and  does  not  pulsate 
as  does  the  plunger  type. 

Ir/nilion. — The  explosive  mixture  in  the  cylinder  is  ignited  at  the 
[jroper  time  by  an  electric  spark  which  jumps  across  the  poles  of 
the  "spark  plug."  This  plug  screws  into  the  cylinder  and  it  is 
important  that  no  escape  of  gas  can  take  place  around  the  outside  or 
inside  of  the  plug.  In  some  bad  types  of  plugs  the  insulation  loosens 
when  the  engine  gets  hot  and  this  causes  bad  compression.  The 
current  which  makes  the  spark  is  supplied  by  a  generator  or  magneto, 
as  described  later. 

Cooling. — Since  the  explosion  takes  place  inside  the  cylinder  itself, 
the  temperature  reached  by  the  gases  is  very  high.  The  cylinder 
walls  have  to  be  cooled  by  some  external  means  in  order  to  prevent 
them  becoming  too  hot. 

The  effects  of  overheating  are: 

1.  The  metal  is  weakened  verj'  considerablj'^  so  that  all  parts  if  not 
cooled  would  have  to  be  verj^  much  thicker  and  heavier  in  order  to 
prevent  distortion  or  fracture. 

2.  The  hibricating  oil  is  burnt  up  and  the  cylinder  scored  by  the 
deposited  carlion;  also  there  is  a  very  great  risk  of  one  or  more  of  the 
pistons  "seizing  up"  in  the  engine  and  stopping  it. 

3.  The  charge  may  explode  or  preignite  during  the  compression 
-troke,  entailing  a  consideral)le  loss  of  power. 

One  of  two  methods  is  usually  employed  for  cooling  the  engine. 
The  alternative  methods  are  (a)  water  cooling,  and  (b)  air  cooling. 

Water  cooling. — ^Jackets  through  which  water  is  kept  circulating 
jare  constructed  around  the  cylinder  walls  and  ends  and  also  around 
the  exhaust- valve  seatings.  These  jackets  are  either  cast,  welded, 
or  electrically  deposited  around  the  cylinders.    The  hot  water  from 


68  AIR  SERVICE  HANDBOOK. 

the  jackets  is  led  to  a  radiator  which  dissipates  the  heat  through  the 
atmosphere.  This  cools  the  water  so  that  it  can  be  used  again.  The 
usual  method  employed  for  circulating  the  water,  especially  with 
fast  running  engines,  is  to  force  the  wafer  through  the  jackets  by 
means  of  a  circulating  pump  driven  by  the  engine  itself. 

In  some  cases  the  water  is  circulated  automatically  on  the  thermo- 
sj'phon  principal,  in  which  the  fact  that  hot  Avater  is  lighter  than 
cold  and  therefore  rises  to  the  top  is  used. 

The  design  of  the  engine  should  admit  of  no  chance  of  air  or  steam 
pockets  forming  which  might  prevent  a  steady  flow  of  water  through 
the  system.  To  prevent  these  pockets  a  small  air  cock  is  usually 
fitted  at  the  highest  points  of  any  bends,  etc.  Small  pipes  are  also 
often  led  from  the  tops  of  all  such  bends  in  the  circulating  water 
piping  to  the  radiator  to  carry  off  any  steam  formed  when  the  engine 
is  running  fast.  Provision  must  be  made  for  completely  draining 
the  jackets  after  using  engines  during  a  spell  of  cold  weather  so  as  to 
avoid  any  chance  of  bursting  the  cylinder  jackets,  pipes,  etc.,  due 
to  the  water  freezing. 

There  are  a  number  <if  compounds  on  the  market  which  can  be 
put  in  the  water  to  prevent  its  freezing,  but  many  of  these  leave  a 
deposit  in  the  radiator  and  so  prevent  it  working  properly. 

All  cooling,  though  very  necessary,  is  extremely  wasteful,  some 
30  to  50  per  cent  of  the  total  heat  given  out  by  the  combustion  of 
the  fuel  being  carried  away  by  it. 

A  small  thermometer  is  attached  to  the  radiator  and  placed  where 
the  pilot  can  see  it  so  that  he  may  know  when  his  engine  is  too  hot 
or  too  cold.  He  can  keep  his  radiator  at  an  even  temperature  by 
opening  or  closing  shutters  in  front  of  it. 

Radiators  are  placed  either  completely  above  the  engine  so  that 
in  case  of  damage  by  a  biillet  they  will  always  have  some  water  in 
them,  or  else  right  in  front  of  the  engine  in  order  to  do  away  with 
head  resistance. 

Air  cooling. — In  many  gasoline  engines  the  cylinders  are  kept 
cool  by  means  of  a  stream  of  air  impinging  on  their  outer  surfaces. 
In  order  to  assist  this  dissipation  of  heat,  gills  or  fms  are  formed 
on  the  outside  of  the  cylinders  which  add  to  their  external  surface 
and  so  increase  the  rapidity  of  heat  diffusion.  WTiere  the  cylinders 
are  small  the  heating  siirface  per  unit  volume  of  cylinder  capacity 
is  sufficiently  large  to  give  cool  running  without  other  means  than 
the  ordinary  rush  of  air  past  the  cylinder  due  to  the  motion  of  the 
body.  The  larger  the  diameter  of  the  cylinder  the  less  will  be  the 
surface  per  unit  volume.  Larger  cylinders  will  therefore  require 
some  additional  means  of  cooling  them  such  as  a  fan  which  forces 
air  into  a  casing  between  the  cylinders.     The  air  escapes  out  of  the 


AIR  SERVICE  HANDBOOK.  69 

casing  past  the  cylinders  and  in  so  doing  cools  them.  In  other 
engines  the  cylinders  are  kept  cool  by  rotating  them  rapidly  through 
the  air. 

One  of  the  chief  difficulties  that  have  to  l)e  contended  with  in  an 
air-cooled  engine  is  the  overheating  of  the  exhaust  valves.  It  is 
sometimes  found  convenient  to  provide  a  water  jacket  round  the 
valve  box  and  stem  guides  to  keep  these  parts  cool.  The  extra 
weight  entailed  is  small  and  is,  on  the  whole,  quite  justified  by  the 
results  obtained .  In  some  engines  the  inlet  valves  are  placed  close  to 
and  opposite  the  exhaust  valves  so  that  the  incoming  mixture 
passes  over  the  exhaust  valves,  thus  tending  to  keep  them  cool. 

VI.  ENGINE   EFFICIENCY. 

The  gasoline  engine  is  simply  a  heat  engine.  It  is  supplied  with 
heat  in  the  form  of  a  fuel  and  each  pound  weight  of  fuel  gives  up  a 
certain  definite  amount  of  heat  when  it  is  completely  burned. 
The  amount  of  heat  in,  say,  1  pound  of  fuel  can  be  accurately 
determined  by  actual  experiment,  1  pound  of  gasoline  being  found 
to  give  up  when  completely  burned  about  22,000  British  thermal 
units  of  heat. 

If  an  auto  tire  be  pumped  up  the  temperature  of  the  pump  barrel 
will  rise,  but  this  is  not  entirely  due  to  friction  inside  the  barrel. 
In  compressing  the  air  work  is  done,  thereby  generating  heat.  If 
now  the  air  is  allowed  to  expand  again  to  its  original  volume  by 
passing  through  a  small  orifice  it  gets  a  high  velocity,  i.  e.,  work  is 
done  by  the  air  on  itself,  and  it  becomes  cool  again.  This  is  exactly 
what  happens  in  the  gasoline  engine.  The  explosion  of  the  mixture 
generates  a  pressure  which  gives  the  heat  a  means  of  doing  work 
and  so  transforming  the  thermal  units  obtained  from  the  explosion 
into  mechanical  work. 

For  every  77<S  foot-pounds  of  work  done  on  the  piston  1  British 
thermal  unit  will  have  to  be  abstracted  from  the  hot  gases  in  the  cylin- 
der to  supply  the  energy  necessary  for  this  work  done. 

The  most  usual  way  of  expressing  the  efficiency  is  as  a  percentage 
of  the  heat  available  in  the  gasoline  used  that  is  turned  into  useful 
work  by  the  engine. 

The  total  heat  received  by  the  engine  is  dissipated  in  four  ways: 

1.  Part  does  useful  work  on  the  piston  in  the  cylinder. 

2.  Part  escapes  with  the  exhaust  gases  during  the  exhaust  stroke. 

3.  Part  goes  into  the  cooling  water  system  (or  to  the  surrounding 
atmosphere  if  the  air-cooling  system  is  used),  provided  to  prevent 
the  cylinders  getting  too  hot  and  so  gets  lost  so  far  as  the  useful 
work  of  the  engine  is  concerned . 

4.  A  very  small  part  is  lost  by  radiation;  this  may  be  neglected 
in  comparison  with  the  losses  due  to  2  and  3. 


70  AIR  SERVICE  HANDBOOK, 

The  efficiency  will  therefore  depend  on  the  magnitude  of  numbers 
2  and  3. 

In  order  that  these  losses  may  be  reduced  to  a  minimum,  i.  e., 
in  order  to  obtain  maximum  efficiency,  it  is  necessary — 

(a)  To  have  the  mixture  of  correct  strength  and  properly  mixed. 

(6)  To  have  as  large  a  ratio  of  expansion  as  possible  consistent 
with  the  avoidance  of  preignition  during  the  compression  stroke. 

(e)  To  advance  the  spark  sufficiently  far  to  insm-e  the  completion  of 
explosion  just  before  the  commencement  of  the  working  stroke. 

(d)  The  temperature  of  the  cooliug-jacket  water  should  be  kept 
as  high  as  possible  consistent  with  cool  running. 

These  conditions  will  now  be  examined  in  detail. 

A.  Too  rich  a  mixture  and  also  too  weak  a  mixture  cause  a  slow 
rate  of  explosion.  The  component  particles  of  the  explosive  charge 
will  not  mix  sufficiently  well  to  get  rapid  combustion;  the  flame 
of  explosion  will  consequently  only  travel  slowly  through  the 
charge.  The  result  is  that  by  the  time  the  explosion  is  finished 
the  piston  will  have  completed  a  considerable  fraction  of  its  work- 
ing stroke  and  the  actual  flame  of  explosion  will  therefore  come 
into  contact  vnth  a  very  large  area  of  the  cylinder  wall.  A  large 
proportion  of  the  heat  will  consequently  be  conducted  through 
the  cylinders  into  the  water  jacket. 

B .  ^Tien  the  piston  is  on  its  working  stroke  it  is  receding  from  the 
gases  in  the  cylinder  head;  the  gases  therefore  expand  and  cool  down. 
Consequently  it  is  necessary  in  order  to  get  them  as  cool  as  possible 
before  being  exhausted  that  the  working  stroke  should  be  as  long  as 
possible.  The  travel  of  the  piston  is  the  same  on  the  compression 
as  on  the  worldng  stroke  and  the  maximum  ratio  of  expansion  is 
practically  the  same  as  the  maximum  ratio  of  compression.  Hence 
it  follows  that  the  maximum  ratio  of  expansion  is  obtained  when  ex- 
plosion is  completed,  just  as  the  piston  starts  on  its  working  stroke. 
At  the  same  time  it  will  be  seen  that  with  this  condition  of  maxiniiun 
ratio  of  expansion  there  will  be  a  minimum  loss  of  heat  to  the  cooling 
system,  for  when  the  gases  are  at  theu-  highest  temperature  (i.  e., 
when  the  explosion  is  just  completed)  they  will  be  in  contact  with 
the  minimum  area  of  cylinder  walls. 

C.  It  must  be  remembered  that  compared  with  the  movement  of 
the  piston  explosion  takes  an  appreciable  time  to  complete.  In 
order  therefore  that  the  explosion  amay  be  completed  by  the  time . 
the  piston  reaches  the  top  of  its  stroke  ignition  must  occur  at  the  latter 
end  of  the  compression  stroke,  i.  e.,  the  spark  must  be  advanced. 
With  the  spark  advanced  and  the  mixture  still  being  compressed 
explosion  takes  place  very  rapidly  and  therefore  the  gases  when  at 
their  highest  temperature  are  in  contact  with  the  cylinder  walls  for 


AIR  SERVICE  HANDBOOK.  71 

the  mijiimum  length  of  time.  If,  however,  the  spark  is  advanced 
too  far;  i.  e.,  with  ignition  taking  place  very  early  in  the  compres- 
sion stroke  explosion  will  be  completed  before  the  piston  reaches 
the  top  of  its  stroke  and  a  very  much  larger  loss  of  heat  to  the  cooling 
system  will  result;  if  the  advance  is  very  excessive  the  explosion 
will  tend  to  prevent  the  piston  from  reaching  the  top  of  its  stroke. 

D.  The  amount  of  heat  conducted  from  the  cylinders  into  the  cool- 
ing system  will  depend  among  other  things  on  the  difference  of  tem- 
perature between  the  cylinders  and  the  cooling  system.  The  larger 
the  difference  the  greater  will  be  the  amount  of  heat  lost. 

Sating  of  weight. — The  saving  of  weight  in  an  aero  engine  is  effected 
by  using  the  strongest  and  lightest  material  and  by  means  of  ihe 
arrangement  of  the  cylinders.  Steel,  which  is  light  for  its  strength 
is  used  nearly  all  through  except  for  those  parts  such  as  the  crank- 
case  cover,  which  is  a  cover  only  and  bears  no  strength. 

In  a  multicylinder  engine  if  the  cylinders  are  put  in  line,  one 
behind  the  other,  the  crank  case  and  crank  shaft  are  very  long  and 
will  have  to  be  constructed  heavily  in  order  to  prevent  undue 
bending.  Each  connecting  rod  also  will  have  to  have  a  bearing  of  its 
own  on  the  crank  shaft.  If  the  cylinders  are  now  arranged  as  is  done 
in  a  V-type  engine  the  weight  of  the  crank  case  and  crank  shaft  will 
be  greatly  reduced  because  they  will  now  be  only  about  half  the 
length  they  were  before.  The  cranks  will  each  take  a  pair  of  con- 
necting rods  so  that  there  is  a  saving  of  weight  in  the  main  bear- 
ings. The  shaft  will  not  have  to  be  any  stronger  because  the  cylin- 
ders will  not  explode  at  the  same  time,  biit  at  different  times.  The 
crank  case  may  be  still  further  reduced  in  size  if  the  engine  is  made 
radial  or  Y-shaped.  This  type  is  used  for  all  rotary  engines.  It  is 
not  altogether  good  for  fixed  cylinder  engines  on  account  of  the  diffi- 
culty in  lubricating  the  bottom  cylinder  or  cylinders  efficiently. 
When  mounted  in  a  machine  also  these  radial  engines  present  a  gi'eat 
head  resistance,  much  more  than  do  the  narrow  V-tj-pe  engines. 

Effect  of  centrifugal  force. — It  is  a  familiar  fact  that  if  a  weight  be 
whii-led  round  at  the  end  of  a  string  it  pulls  outward  and  the  force 
with  which  it  pulls  is  described  as  centrifugal  force.  A  striking 
example  of  centrifugal  force  is  afforded  by  the  case  of  a  bucket  of 
water  which  may  ])e  swung  around  in  the  same  way  as  a  weight  with- 
out spilling  the  water.  These  facts  are  common  knowledge,  but  it 
is  not  generally  realized  that  the  forces  may  be  of  such  a  magnitude 
that  they  have  to  be  taken  into  consideration  in  the  design  of  engines 
and  in  fact  frequently  form  the  controlling  factor.  As  an  example, 
we  may  take  the  case  of  the  80-horsepower  Gnome  engine,  an  engine 
of  the  rotary  type,  which  has  a  diameter  of  approximately  2  feet  8 
inches.     When   running  at   its  full   speed    (1.200   revolutions  per 


72  AIR  SERVICE  HANDBOOK. 

minute)  each  pound  of  metal  at  the  cylinder  heady  is  pulling  outward 
with  a  force  of  65  pounds. 

The  existence  of  this  large  outward  force  gives  rise  to  difficulties 
in  connection  with  engine  designs  and  in  particular  in  connection 
with  the  valve  gear  of  rotary  engines.  In  some  engines  this  force 
is  neutralized  by  the  fitting  of  balance  weights,  in  others  the  valves 
have  to  be  made  especailly  heavy  to  balance  the  tappet  rods.  It  is 
very  helpful  in  one  respect  and  that  is  in  connection  with  lubrica- 
tion. In  all  rotary  engines  the  lubricating  oil,  is  pressure  fed  into  a 
duct  or  ducts  inside  the  metal  of  the  crank  shaft.  From  this  duct 
holes  lead  to  the  bearings,  etc.,  and  the  oil  from  these  parts  is  carried 
outward  by  centrifugal  force  to  the  outer  parts  of  the  engine.  In 
rotary  engines  the  surplus  oil  reaches  the  cylinder  heads  and  is  blown 
out  with  the  exhaust. 

In  stationary  engines  the  rotating  cranks  and  crank  pins  are  bal- 
anced in  some  cases  by  balance  weights  and  in  some  cases  by  other 
cranks.  The  connecting  rods  and  piston  of  stationary  engines  also 
require  balancing  and  in  the  case  of  multicylinder  engines  it  is 
possible  and  usual  to  arrange  that  the  movements  of  the  different 
pistons  and  connecting  rods  are  such  as  to  more  or  less  completely 
balance  each  other. 

Lightness  in  reciprocating  parts. — Consider  a  single-cylinder  engine 
driven  around  by  some  ext"rnal  moans  such  as  an  electric  motor. 
The  piston  is  pulled  and  pushed  on  the  connecting  rod  at  the  top  and 
bottom  of  the  stroke.  The  result  is  that  there  is  a  considerable  un- 
balanced force  exerted  on  the  crank  pin  at  these  points.  The  up- 
ward and  downward  motion  of  the  piston  gives  rise  to  vibration  of 
the  engine  which  at  high  speeds  would  be  very  marked  if  not  bal- 
anced. In  a  single-cylinder  engine  this  can  be  done  only  by  the 
addition  of  extra  balance  weights  opposite  the  crank  throw.  This, 
however,  results  in  the  introduction  of  new,  tmbalanced  forces  at 
right  angles  to  the  original  unbalanced  forces,  and  it  is  therefore  not 
practicable  to  completely  balance  the  piston  of  a  single-cylinder 
engine. 

The  reciprocating  parts  of  an  engine  are  always  made  as  light  as 
possible  in  order  that  the  forces  to  be  balanced  shall  be  reduced  to 
a  minimum.  A  heavy  piston,  for  instance,  will  take  a  considerable 
force  to  raise  it  quickly  up  the  cylinder  and  when 'it  gets  to  the 
top  it  will  again  recjuire  a  considerable  force  to  stop  it  going  up  and 
to  make  it  come  down. 

It  must  be  remembered  that,  although  the  engine  as  a  whole  may 
be  perfectly  balanced,  the  forces  exist  within  the  engine  and  result 
in  extra  stresses  and  wear  of  the  parts  through  which  the  stresses  are 
transmitted. 


AIR  SERVICE   HANDBOOK.  73 

Engine  speeds. — A  pilot  must  know,  amongst  other  things,  the 
normal  running  speed  of  his  engine.  The  rotary  engines  run  at 
about  1,200  revolutions  per  minute.  The  stationary  engines  in 
general  run  at  higher  speeds  and  in  some  cases  the  propeller  is  driven 
through  a  speed-reducing  gear. 

As  a  general  rule  the  larger  engines  of  a  given  class  run  at  lower 
speeds,  but  the  differences  in  speed  between  aero  engines  of  different 
powers  is  small  compared  with  the  differences  in  other  types  of 
engines,  owing  to  the  fact  that  in  aero  engines  extra  power  is  mainly 
obtained  by  the  adding  of  extra  cylinders,  whereas  in  the  case  of 
ordinary  gas,  oil,  and  steam  engines  extra  power  is  mainly  obtained 
by  increasing  the  size  of  the  cylinders.  In  other  words,  large  cylin- 
ders involve  low  speeds  and  small  cylinders  permit  of  high  speeds. 

It  may  be  assumed  as  a  broad  guiding  principle  that  "the  higher 
the  speed  the  smaller  the  engine  will  be  for  a  given  power." 

The  power  given  l)y  any  one  engine  is  proportional  to  the  engine's 
speed;  that  is,  an  engine  run  at  half  its  normal  speed  \^all  give  half 
its  normal  power  (within  practical  limits).  In  the  case  of  aero 
engines  it  is  of  course  of  the  greatest  importance  to  get  the  most 
power  possible  from  a  given  engine,  hence  speeds  are  made  as  high 
as  practically  possible. 

The  disadvantages  of  high  speeds  are: 

1.  Extra  vibration. 

2.  Extra  wear  of  engine. 

3.  Extra  stresses  in  the  engine. 

4.  Greater  difficulties  in  lubrication. 

5.  Greater  difficulties  in  cooling. 

Thus  it  is  seen  that  engines  should  never  be  run  at  maximum  for 
long  periods  unless  there  is  some  very  urgent  reason. 

It  will  be  found  that  if  the  speed  of  an  engine  is  increased  above  a 
certain  limit  the  power  given  by  the  engine  no  longer  increases  and 
ih  fact  falls  off  with  a  further  increase  of  speed.     This  is  due  to — 

(a)  The  valves  failing  to  follow  the  cams  at  very  high  speeds;  and 

(b)  Reduction  in  the  volume  of  the  charge  drawTi  in  through  the 
inlet  and  ejected  through  the  exhaust  owing  to  the  very  short  time 
the  valves  are  open. 

In  case  («)  it  may  be  thought  that  the  difficulty  might  bo  over- 
come by  fitting  stronger  valve  springs.  Such  a  method  is  not 
practical.  It  results  in  almormal  wear  of  the  A^alve  gear  and  frac- 
tured valves.  The  correct  solution  of  the  difficulty  lies  in  the 
direction  of  lightening  the  valves,  tappets,  etc.,  and  reducing  the 
inertia  of  the  moving  parts.  In  some  engines  there  are  two  or  more 
inlet  and  exhaust  valves  in  the  cylinder  head  for  this  reason.  In 
some  engines  also  the  cam  shafts  are  mounted  over  the  cylinders  so 


74  AIR  SERVICE  HANDBOOK. 

that  tapper  rods  can  be  done  away  with.  The  valves  are  made  as 
light  as  possible  and  each  is  operated  through  a  short  rocker  arm  by 
its  own  cam. 

Propeller  speeds. — The  question  of  propeller  and  engine  speeds  in 
aircraft  is  roughly  analagous  to  the  corresponding  problem  in  marine 
propulsion.  The  most  efficient  propeller  runs  at  a  comparatively 
low  speed,  and  there  is  a  certain  difficulty  in  gearing  the  propeller 
to  the  engine  without  loosing  efficiency. 

In  the  case  of  rotary  aircraft  engines  where  the  propellers  are 
direct  driven  the  speed  (1,200  revolutions  per  minute)  is  too  high 
for  the  most  efficient  propeller,  and  those  used  on  rotary  engines  are 
made  with  a  finer  pitch  than  that  required  for  best  efficiency. 

In  some  stationary  aero  engines  the  propellers  are  direct  driven, 
but  in  the  majority  of  cases  they  are  driven  at  a  lower  speed  than  the 
engine.  This  is  effected  by  running  the  propeller  off  the  cam  shaft, 
which  is  rotating  at  half  the  engine  speed,  or  else  by  means  of  a 
special  reduction  gear.  It  should  be  noted  that  a  speed -reduction 
gear  uses  up  and  wastes  a  certain  amount  of  power  and  it  also  adds 
weight  to  the  engine. 

Another  point  is  worth  considering.  As  is  well  known  the  pro- 
peller torque  or  twisting  force  reacts  upon  the  airplane  and  tends  to 
turn  it  over  sideways.  This  tendency  is  counteracted  by  wash  in 
and  wash  out  on  the  wings  of  the  machine.  If  the  propeller  speed 
is  high  foT  a  given  power  the  torque  is  small  and  this  effect  is  least, 
but  if  the  speed  is  low  the  effect  will  be  maximum. 

Direction  of  rotation. — Normal  engines  rotate  in  a  counterclock- 
wise direction  as  seen  from  the  propeller  end,  i.  e.,  the  engines  are 
right-handed.  In  engines  with  speed-reducing  gear,  with  some 
exceptions,  the  propeller  rotates  the  opposite  way  and  the  engine 
left-handed. 

VII.  PROPELLERS. 

The  sole  object  of  the  propeller  is  to  produce  tlu'ust.  The  thru^ 
overcomes  the  drift  of  the  airplane  and  draws  (or  pushes)  the  air- 
plane through  the  air. 

The  thrust  must  be  e;|ual  to  the  drift  of  the  airplane  at  flying 
speed.  If  it  is  not  equal  to  the  drift,  then  the  airi:)lane  can  not 
secure  its  proper  speed. 

The  thrust  will  be  badly  affected  if  any  of  the  following  condi- 
tions are  not  as  they  should  be: 

Pitch-angle. — The  propeller  screws  through  the  air,  and  its  blades 
are  therefore  set  at  an  angle.  This  angle  is  known  as  the  pitch-angle, 
and  must  be  correct  to  half  a  degi'ee.  It  is  of  course  smaller  toward 
the  tips  of  the  blades,  just  as  in  the  case  of  the  pitch-angle  of  a 
marine  propeller. 


AIR  SERVICE  HANDBOOK. 


76 


Pitch. — The  pitch  is  the  distance  the  propeller  will  advance 
through  the  air  in  one  revolution,  supposing  the  air  to  be  solid.  As 
a  matter  of  fact,  the  air  is  not  solid  and  gives  back  to  the  thrust  of 
the  propeller  blades,  so  that  the  propeller  does  not  travel  its  full 
pitch.  Such  "give  back"  is  known  as  slip.  For  instance,  the 
pitch  of  the  propeller  may  be  perhaps  10  feet  and  the  propeller  may 
have  a  slip  of  2  feet.  The  propeller  would  thejjjse  5aidrr*<Lhave 
20  per ''""+ °i''"  -^     ^  *-'' 


To  test  the  pitch-angle  the  propeller  is  mounted  on  a  shaft,  the 
latter  being  mounted  upon  and  at  right  angles  to  a  beam.  The 
face  of  the  beam  must  be  perfectly  straight  and  true. 

Now  select  a  spot  some  distance  (say  about  2  feet)  from  the 
center  of  the  propeller  and  by  means  of  a  protractor  find  the  angle 
the  chord  of  the  blade  makes  with  the  beam.  Then  lay  out  the 
angle  oft  paper,  thus: 


Fig.  26. 

The  line  marked  "Chord"  represents  the  chord  of  the  propeller. 
The  line  marked  "Circumference"  represents  the  face  of  the  beam. 
The  angle  the  two  lines  make  is  the  angle  you  have  found  by  means 
of  the  protractor. 

We  will  suppose,  for  the  sake  of  example,  that  the  point  at  which 
you  have  taken  the  angle  is  2  feet  fi-om  the  center  of  the  propeller. 
Find  the  circumference  at  that  point  by  doubling  the  2  feet  (which 
is  the  radius)  and  then  multiplying  the  result  by  3.1416,  thus 
(2'+2)X3. 1416=12.5668';  i.  e.,  the  circumference  at  that  part  of  the 
propeller. 

Bring  it  down  in  scale  and  mark  it  off  from  the  point  A  and  along 
the  circumference  line.  Now  draw  the  line  marked  "Pitch"  from 
B  (the  end  of  the  circumference  measurement  of  12.5668'')  and  at 
right  angles  to  the  circumference  lino. 


76  AIR  SERVICE  HANDBOOK. 

The  distance  from  B  to  the  chord  line  is  the  pitch  of  the  propeller 
at  that  point. 

It  must  agree  with  the  specified  pitch  of  the  propeller,  which 
should  be  marked  on  the  hub.  If  it  does  not  do  so,  then  the  pitch- 
angle  is  wrong.    This  may  be  due  to— 

1.  The  propeller  blade  being  distorted. 

2.  To  faulty  manufacture. 

3.  To  the  hole  through  the  propeller  boss  being  out  of  place. 
Degree  of  error  allowed. — An  error  up  to  half  a  degree,  more  or  less, 

from  the  correct  angle  may  be  allowed,  but  if  it  is  greatei  the  matter 
should  be  reported  and  the  propeller  changed.  The  jiropeller 
should  be  tested  as  explained  above  at  points  along  the  blades,  the 
first  point  about  2  feat  from  the  center  of  the  box  and  the  others 
about  a  foot  apart. 

Length.— l^\ie  propeller  should  be  carefully  tested  to  make  sure 
the  blades  are  of  equal  length.  There  should  not  be  a  difference  of 
more  than  seven-sixteenths  of  an  inch. 

\Sfi<3/t  or?  3a//  3eari  nys. 

Wet  fTft  i  n  Bo/rNo/e 


Fig. 


Balance  .—'Yh^  prevailing  method  for  testing  for  balance  is  as 
follows:  Mount  the  propeller  on  a  shaft.  The  shaft  must  be  on  ball 
bearings.  Place  the  propeller  in  a  horizontal  position,  and  it  should 
remain  in  that  position.  The  propeller  should  also  remain  station- 
ary when  placed  in  a  vertical  position. 

If  a  weight  of  a  trifle  over  an  ounce  placed  in  a  bolt  hole  on  one 
side  of  the  hub  fails  to  disturb  the  balance,  then  the  propeller  is 
unfit  for  use. 

The  above  method  does  not,  however,  test  for  the  balance  of 
centrifugal  force,  which  comes  into  play  as  soon  as  the  propeller 
revolves.    The  test  for  centrifugal  balance  is  as  follows: 

The  propeller  must  be  kept  hori?ontal,  and  while  in  that  position 
weighed  at  any  fixed  points,  such  as  A,  B,  C,  D,  E,  and  F,  and  the 
weights  noted.  Now  reverse  the  propeller  and  weigh  at  each  point 
again.  Note  the  results.  The  distances  of  corresponding  points  on 
either  side  of  the  center  of  the  hub  should  be  equal.  The  first 
series  of  weights  should  correspond  to  the  second  series,  thus:  Weight 
A  should  equal  weight  F;  weight  B  should  equal  weight  E;  weight 
C  should  equal  weight  D. 


AIR  SERVICE  HANDBOOK. 


77 


There  is  no  official  ruliiij^  as  to  the  degree  of  error  allowed,  but  if 
there  is  any  appreciable  difference  the  propeller  is  unlit  for  use. 

Surface  area. — The  surface  area  of  the  blades  should  be  equal. 
Test  with  callipers.     (See  (ig.  27.) 

The  distance  A-B  should  equal  K-L;  the  distance  C-D  should 
equal  I-J;  the  distance  E-F  should  equal  G-H. 

There  is  no  official  ruling  as  to  the  degree  of  error  allowed;  if, 
however,  there  is  an  error  of  over  one-eighth  inch,  the  propeller 
is  really  unfit  for  use.     The  corresponding  points  on  each  side  of 


Fig.  28. 

the  propeller  must,  of  course,  be  exactly  the  same  distance  from  the 
center  of  the  propeller. 

Camber  (i.  e.,  curvature). — ^The  camber  of  the  blades  should — 

1.  Be  equal; 

2.  Should  decrease  evenly  toward  the  tips  of  the  blades;  and 

3.  Its  greatest  depth  should  at  any  point  of  the  blade  be  at  about 
the  same  proportion  of  the  chord  from  the  leading  edge  as  at  other 
points. 

It  is  difficult  to  test  the  top  camber  without  a  set  of  templates, 
but  a  fairly  accurate  idea  of  the  curvature  underneath  the  blade 


Fig.  29. 

can  be  secured  by  slowly  passing  a  straightedge  along  the  blade, 
the  straightedge  (a  steel  rule  will  do)  being  held  at  right  angles 
to  the  length  of  the  blade  and  touching  both  leading  and  trailing 
edges,  thus — 

The  concave  curvature  can  now  be  easily  seen,  and  as  you  pass 
the  straightedge  along  the  blade  you  should  look  out  for  any  irregu- 
larities of  the  curvature,  wliich  should  gradually  and  evenly  decrease 
toward  the  tip  of  the  blade. 

Straiglitnfss. — To  test  for  straightness  mount  the  propeller  upon 
a  shaft.  Now  bring  the  tip  of  one  blade  round  to  graze  some  fixed 
object.     Mark  the  point  it  grazes.     Now  bring  the  other  tip  round 


78  AIR  SERVICE  HANDBOOK. 

and  it  should  come  within  one-eighth  inch  of  the  mark.  If  it  does 
not  do  so  it  is  due  to  the  propeller  being  distorted  or  to  the  hole 
through  the  boss  being  out  of  place.     In  either  case  it  is  unfit  for  use. 

The  joints. — The  method  for  testing  the  glued  joints  is  by  revolving 
the  propeller  at  5  to  10  per  cent  greater  speed  than  it  will  be  called 
upon  to  make  in  flight  and  then  carefully  examining  the  joints 
to  see  if  they  have  opened.  It  is  not  likely,  however,  that  you 
will  have  the  opportunity  of  making  this  test.  Yoii  should ,  however, 
examine  all  glued  joints  very  carefully,  trying  by  hand  to  see  if 
they  are  quite  sound.  Suspect  a  propeller  in  which  the  joints 
appear  to  hold  any  thickness  of  glue.  Sometimes  the  joints  in  the 
boss  open  a  little,  but  this  is  not  dangerous  unless  they  extend  to 
the  blades,  as  the  bolts  will  hold  them  together. 
■  Condition  of  surface. — The  surface  should  be  perfectly  smooth 
especially  toward  the  tips  of  the  blades.  Some  propeller  tips  have 
a  speed  of  over  30,000  feet  a  minute  and  any  roughness  will  produce 
a  bad  drift  or  resistance  and  spoil  the  efficiency  of  the  propeller. 

Long  grass  on  the  airdrome  and  mud  and  water  will  take  all  the 
polish  off  the  propeller  tips  and  sometimes  do  serious  damage. 

Mounting  -propeller. — Be  careful  to  see  that  the  propeller  is 
mounted  quite  straight  on  its  shaft.  After  mounting  test  it  the  same 
way  as  was  done  for  straightness.  Before  reporting  the  propeller 
as  faulty  make  certain  that  the  bolts  have  been  screwed  up  equally 
all  round  and  that  some  are  not  too  loose  or  too  tight. 

Care  of  propellers. — The  care  of  propellers  is  of  the  greatest  im- 
portance, as  they  are  very  likely  to  distort  and  loose  their  correct 
pitch-angle  and  straightness. 

1.  Do  not  store  propellers  in  a  very  damp  or  very  dry  place. 

2.  Do  not  store  them  where  the  sun  will  shine  on  them. 

3.  Never  leave  them  in  a  horizontal  position  or  leaning  up  against 
a  wall. 

4.  They  should  be  hung  on  pegs,  the  latter  at  right  angles  to  the 
wall  and  the  position  of  the  propeller  should  be  vertical. 

If  the  points  noted  above  are  not  attended  to  you  may  be  sure 
of  the  following  results: 

1.  Lack  of  efficiency,  resulting  in  less  airplane  speed  and  climb 
than  would  otherwise  be  the  case. 

2.  Propeller  "flutter;"  i.  e.,  vibration,  which  will  cause  the 
propeller  to  distort  and  possibly  collapse. 

3.  A  bad  stress  upon  the  crank  shaft  and  its  bearings. 
Swinging   the   propeller. — Before  swinging  the   propeller   (i.    e., 

rotating  it  to  start  the  engine)  it  is  necessary  to  know  in  which 
direction  it  should  be  turned.  Unless  the  propeller  has  been  fitted 
to  the  engine  incorrectly  it  is  quite  easy  to  see  which  way  to  turn  it. 


AIR   SERVICE   HANDBOOK.  79 

Grasp  the  trailing  edge  of  the  blade;  the  trailing  edge  is  much 
thinner  than  the  leading  edge.  Move  the  propeller  slightly  so  that 
you  may  "feel"  the  compression.  The  rotation  of  the  propeller 
will  now  be  in  such  a  direction  that  the  flattest  side  of  the  blade  will 
engage  the  air  and  press  against  it. 

As  a  rule  when  the  propeller  is  fitted  to  the  crank  shaft  it  revolves 
in  an  anticlockwise  direction  when  viewing  it  from  the  position 
you  stand  in  to  swing  it. 

When  it  is  fitted  to  another  shaft  which  is  geared  to  a  crank  shaft, 
then,  as  a  rule,  it  revolves  in  a  clockwise  direction. 

VIII.  STARTING  THE  ENGINE. 

Sound  footing. — First  of  all  make  sure  that  the  ground  just  in  front 
of  the  propeller  affords  you  a  good  sound  footing.  Should  your  foot 
slip  when  swinging  the  propeller  the  result  may  be  very  bad  for 
yourself. 

Now  place  the  blocks  in  front  of  the  wheels  of  the  machine  and 
lay  out  their  cords  toward  the  wing  tips.  These  blocks  should 
always  have  cords  attached ;  otherwise  sooner  or  later  a  mechanic 
will  step  into  the  propeller  when  it  is  revolving. 

One  mechanic  should  be  at  each  wingtip  and  should  grasp  the  bot- 
tom of  the  outer  struts  to  steady  the  airplane  when  the  engine  is 
running.  They  should  not  hold  the  leading  edge  of  the  plane,  be- 
cause if  they  do  they  will  probably  leave  oily  marks  on  the  fabric, 
and  the  dope  will  slack  off  in  these  places.  These  mechanics  will 
pull  the  blocks  away  when  the  pilot  signals  for  such  action. 

There  should  be  not  less  than  two  mechanics  at  the  tail  end  of  the 
fuselage  in  order  to  keep  it  down  when  the  engine  is  running.  In 
many  machines  the  tail  may  be  kept  down  by  pulling  the  control 
lever  well  in  toward  one. 

Rotary  engines. — In  the  case  of  some  rotary  engines  it  is  necessary, 
after  ascertaining  that  the  switch  is  off,  to  prime  the  cylinders  with 
gasoline.  This  is  done  by  squirting  gasoline  through  the  exhaust 
port.  Great  care  should  be  exercised  to  make  sure  that  the  squirt  is 
clean.  Never  lay  it  on  the  ground.  The  top  of  a  gasoline  tin  is  a 
good  and  convenient  place. 

Switch  off. — Before  attempting  to  rotate  the  propeller  always  make 
sure  that  the  ignition  switch  is  off.  Otherwise  the  engine  and  pro- 
peller may  start  unexpectedly  with  disastrous  results  to  the  starter. 
There  has  been  more  than  one  fatal  accident  due  to  carelessness  in 
overlooking  this  point.  Never  touch  a  propeller  without  saying 
"switch  off."    Always  see  that  the  ground  wire  is  connected. 

Gasoline  on  and  air  dosed. — Now  ascertain  that  the  gasoline  is 
on  and  the  air  closed.    The  air  is  not  really  quite  closed,  but  is  partly 


80  AIB,  SERVICE  HANDBOOK. 

cut  off  80  that  the  mixture  may  be  rich  in  gasoline  in  order  to  facili- 
tate the  first  few  explosions. 

Rotate  propeller. — Now  swing  the  propeller  round.  This  will  turn 
the  engine,  and  the  effect  of  the  descending  pistons  will  be  to  suck 
the  mixtui'e  into  the  cylinders. 

Contact. — Now  sing  out  "on'*'  to  the  pilot.  He  will  put  the  igni- 
tion "on-'  repljdng  to  you  "on." 

Swing  propeller. — Now  one  good  downward  swing  of  the  propeller 
blade  and  stand  clear.  If  the  engine  fails  to  start,  ask  the  pilot  to 
"switch  off'*'  and  go  through  the  same  operation  again. 

Starting  magneto. — In  some  engines  a  starting  magneto  may  be 
used  instead  of  swinging  the  propeller,  in  which  case,  after  sucking 
in  the  mixture,  the  propeller  should  be  left  in  such  a  manner  that  one 
cylinder  is  ready  to  fire.  This  can  be  felt  by  gently  moving  the 
propeller  up  and  down. 

Self-starters. — If  a  self-starter  is  used  it  is  done  in  the  same  manner 
as  is  done  on  a  car.  In  small,  fast  machines  the  use  of  the  self- 
starter  does  not  repay  the  loss  of  efficiency  due  to  weight. 

Danger  of  '"kicking  back." — ^When  swinging  the  propeller  be  care- 
ful to  stand  clear  of  it.  There  is  often  a  possibility  of  the  engine 
"kicking  back^'  and  suddenly  turning  the  propeller  the  wrong  way 
around.  This  is  usually  due  to  ignition  occurring  early,  i.  e.,  before 
the  piston  arrives  at  the  top  of  the  cylinder,  and  if  the  engine  is 
revolving  slowly  the  momentum  of  the  moving  parts  (crankshaft, 
propeller,  etc.)  may  not  be  sufficient  to  carry  it  round  in  the  right 
direction.  The  result  of  tliis  is  that  the  piston  never  gets  to  the  top 
of  its  stroke  but  descends  again,  driving  the  crank  shaft  back  and 
round  in  the  wrong  direction.  For  this  reason  when  you  have  de- 
cided to  swing  the  propeller  give  it  one  good  downward  swing.  Do 
not  give  any  small  preliminary  swings  after  you  have  called  "on.'' 

Signals. — 1.  The  pilot  when  ready  to  start  will  wave  his  hand 
from  side  to  side.  This  is  the  signal  for  the  bloclvs  under  the  wheels 
to  be  pulled  away  smartly  by  means  of  the  cords  attached  to  them. 

2.  Now  the  pilot  waves  his  hand  in  a  fore  and  aft  direction.  This 
is  the  signal  for  everyone  to  stand  clear  without  a  moment's  delay, 
and  is  especially  meant  for  the  mechanics  at  the  tail  of  the  fuselage. 

While  the  pilot  has  his  hand  still  raised  he  gives  a  look  around 
to  see  that  everything  is  clear.  lie  then  taxis  to  the  place  from 
where  he  is  going  to  start. 

IX.  DEFECTS   IN   THE   ENGINE. 

Defects  in  the  engine,  their  causes  and  remedies. — There  are  three 
essential  conditions  to  be  fulfilled  in  order  that  an  internal  com- 
bustion engine  may  be  started  and  then  be  able  to  carry  on  work- 
ing.   These  are: 


AIR  SERVICE  HANDBOOK.  81 

1.  The  mixture  imisl  l)e  of  the  correct  strength  and  its  components 
properly  mixed. 

2.  There  must  be  sutticient  com])ressioii  to  l)riaji;  particles  of  the 
mixture  into  intimate  contact  with  each  other  and  so  render  them 
explosive. 

3.  There  must  be  some  method  of  igniting  the  charge  at  the 
right  time  of  the  stroke,  i.  e.,  somewhere  near  the  end  of  the  com- 
pression. 

If  the  engine  refuses  to  start  after  flooding  the  carbureter,  opening 
the  air  inlet,  and  smtching  on  the  current,  or  putting  other  igni- 
tion devices  into  gear,  etc.,  proceed  to  test,  if  all  these  conditions 
are  being  fulfilled,  as  follows: 

1.  Faulty  mixture. — A  frequent  trouble  with  the  mixture  is  due 
to  omission  to  turn  on  the  gasoline  or  to  the  gasoline  pipe  being 
choked.  Examine  the  gasoline  pipe  and  filter  and  clean  if  nec- 
essary. If  gasoline  be  pressure  fed,  there  may  be  insufficient 
pressure  in  the  tank  to  force  the  gasoline  into  the  float  chamber  of 
the  carbureter.  In  any  case  of  the  engine  refusing  to  start  or  sud- 
denly stopping  always  first  look  to  the  gasoline  supply  by  ascer- 
taining if  the  carbureter  will  flood.  A  choked  spray  or  water  in 
the  carbureter  may  also  be  the  cause  of  the  trouble.  Examine  and 
blow  through  the  jet;  remove  the  float  and  see  if  any  water  or  dirt 
is  present  at  the  bottom  of  the  float  chamber;  if  so,  drain  it  out. 
A  piece  of  copper  tubing  fitted  too  tightly  into  an  india-rubber 
connecting  pipe  may  fray  the  rubber  and  so  cause  the  pipe  to  be- 
come choked.  When  starting  the  engine  by  hand,  it  is  often  nec- 
essary to  flood  the  carbureter  so  as  to  insure  gasoline  coming  into 
contact  with  the  air  going  to  the  cylinders  past  the  top  of  the  jet, 
or  else  to  squirt  some  gasoline  through  the  compression  cocks  into 
the  cylinders  before  starting.  This  latter  is  termed  "priming  the 
cylinders. ' '  In  cold  weather  less  air  is  required,  since  the  increased 
density  of  the  air  allows  a  greater  weight  to  pass  through  the  air 
inlet.  Failure  to  close  the  extra  air  inlet  before  trying  to  start 
will  very  often  prevent  the  engine  from  starting.  When  starting 
up  with  a  cold  carbureter  in  cold  weather  the  gasoline  will  tend 
to  condense  in  the  intake  pipe;  a  rag  wrapped  round  the  inlet 
pipe  will  be  of  assistance  in  getting  the  engine  under  way.  The 
needle  valve  in  the  float  chamber  may  be  warmed  so  that  the  float 
has  to  be  raised  above  its  normal  level  before  it  allows  the  needle 
valve  to  drop  and  shut  off  the  gasoline  supply.  The  needle  valve 
itself  may  also  leak  due  to  wear  or  dii't  lying  on  its  seating.  Both 
of  these  will  give  too  high  a  level  of  gasoline  in  the  float  chamber 
and  thus  in  the  jet;  hence  the  mixture  will  become  too  rich  in 

46643—18 6 


82  AIR  SERVICE  HANDBOOK. 

gasoline.  If  excessive  the  carbureter  float  chamber  will  overflow. 
The  float  may  be  leaky  and  hence  become  too  heavy;  this  if  only 
a  small  amount  of  gasoline  has  leaked  into  it  will  produce  the  same 
results  as  above.  If  much  has  entered,  the  float  will  not  shut  the 
needle  valve  at  all  and  the  carbureter  will  therefore  flood  badly 
and  overflow.  For  this  reason  in  engines  where  a  flame  is  highly 
dangerous  the  overflow  and  all  drains  should  be  led  to  a  funnel 
well  away  from  the  carbureter,  so  that  a  back  fire  or  flash  back 
will  not  cause  a  conflagration.  Flooding  of  the  carbureter  may 
also  be  caused  by  the  upper  part  of  the  needle  valve  being  too 
neat  a  fit  in  its  guide,  or  by  the  needle  valve  being  too  light  for 
its  work  and  so  unable  to  shut  on  its  seating  against  the  head  of 
gasoline.  A  piece  of  waste  left  in  the  air- inlet  pipe  after  an  over- 
haul is  by  no  means  an  uncommon  cause  of  failure  of  an  engine 
o  start. 

2.  Compression. — If  the  engine  is  turned  by  hand  and  the  com- 
pression cocks  at  the  top  of  the  cylinders  are  open  during  the  com- 
pression stroke,  aii-  will  be  forced  out  of  these  cocks  at  high  speed  if 
the  degree  of  compression  is  anything  like  good.  It  should  be  noted 
that  small  leaks  in  the  inlet  and  exhaust  valves,  sparking  plug 
terminals,  etc.,  will  not  appreciably  affect  compression  when  actu- 
ally running  owing  to  the  very  small  fraction  of  a  second  occu- 
pied by  the  compression  stroke,  though,  when  testing  by  "hand  revo- 
lution" the  effect  may  be  very  marked  especially  in  large  cylin- 
ders. The  power  required  to  turn  the  engine  roxmd  by  hand  will 
also  indicate  roughly  the  compression  in  the  different  cylinders. 
Should  the  compression  be  bad  examine  for  the  following  faults: 

A.  Leaky  or  broken  piston  rings.  The  piston  rings  should  bear 
against  the  side  of  the  cylinder  over  their  whole  cii'cumference. 
When  testing  for  this  the  cylinder  walls  should  be  covered  \A-ith  a 
thin  coat  of  rouge  or  red  lead  and  the  piston  rings  put  inside  and 
moved  up  and  down.  The  marking  on  the  rings  will  indicate 
whether  they  are  bedding  properly  and  also  is  there  is  insufiicient 
spring  left  in  the  rings.  When  fitting  new  rings  allow  one  thirty- 
second  or  one  sixteenth  inch  space  between  the  butts  of  these  rings 
when  in  the  cylinder  to  allow  for  expansion.  The  rings  maj^  be 
gummed  into  their  slots  in  the  pistons  by  foul  oil .  Kerosene  injected 
into  the  cylinder  and  left  for  a  few  minutes  will  dissolve  the  oil  and 
release  the  rings. 

B.  The  valve  and  valve  seat  may  be  pitted  or  worn.  The  seat  in 
the  cylinder  and  the  beveled  edge  on  the  valve  should  be  gi-ound 
together  \\'ith  emery  powder,  coarse  powder  being  used  to  start  with 
and  the  very  finest  when  finishing.  If  a  gi'oove  has  been  cut  on  the 
valve  during  the  process  of  grinding  in  skim  the  valve  up  in  a  lathe 


AIR  SERVICE  HANDBOOK.  83 

and  then  finish  off  by  grinding  into  place  with  very  fine  emery 
powder.  The  valve  or  its  seat  may  be  warped  as  a  result  of  over- 
heating; if  only  slightly  grind  in  as  above:  if  very  bad  new  valves 
vn\l  have  to  be  fitted  from  the  spares  and  the  seat  will  have  to  be 
reseated.  The  valve  spindles  may  be  too  tight  a  fit  in  their  guides 
due  to  too  big  a  valve  stem,  or  the  presence  of  oil  and  carbon  in  the 
valve  guide.  Kerosene  will  clean  the  latter.  If  the  valve  stem 
is  at  fault  a  rub  with  emery  cloth  will  often  give  sufficient  slackness. 
Insufficient  or  no  clearance  may  have  been  left  between  the  valve 
stem  and  the  tappet.  This  is  easily  adjusted,  once  discovered. 
The  timing  of  the  cam  shaft  may  be  WTOng.  This  must  be  checked. 
When  checking  take  all  settings  with  the  engine  moving  in  the 
ahead  direction  only. 

3.  Ignition. — Failure  of  electrical  arrangements  are  dealt  with 
under  "magneto." 

4.  Back  firing. — This  occurs  when  the  charge  explodes,  immedi- 
^ately  it  enters  the  cylinder  through  the  open  valves  and  back  fires 

into  the  intake  manifold  and  so  to  the  carbureter  which  it  is  liable 
to  set  alight  if  there  be  any  gasoline  in  it.     The  causes  of  back  firing: 

A.  The  most  common  cause  is  through  a  very  weak  mixtm-e  being 
supplied  to  the  cylinder.  The  charge  does  not  explode  and  is  still 
burning  when  the  inlet  valve  opens  on  the  next  inlet  stroke.  This 
may  be  due  to  the  gasoline  supply  cock  being  only  partly  open,  or 
the  strainer  or  jet  becoming  choked  by  grit,  dirt,  etc. 

B.  Carbon  deposits  get  formed  on  piston  heads  and  walls  of  the 
compression  space  if  the  mixture  supplied  be  constantly  too  rich  in 
gasoline;  this  carbon  cakes  and  becomes  heated  to  incandescence 
and  so  ignites  the  incoming  charge  immediately  on  contact  taking 
place. 

('.  A  leaky  exhaust  valve,  a  weak  spring,  or  a  sticky  spindle  on  an 
exhaust  valve  would  allow  hot,  exhaust  gas  to  be  sucked  in  from  the 
exhaust  pipe  during  the  suction  stroke  and  mix  with  the  incoming 
charge  which  it  raises  to  ignition  temperature  causing  a  premature 
explosion. 

D.  Water  in  the  gasoline  will  sometimes  be  the  means  of  producing 
a  back  fire  due  to  the  same  cause  as  "A." 

E.  Electric  ignition  leads  not  being  joined  up  correctly  or  one  of 
the  high-tension  leads  making  electric  contact  with  another  and 
causing  a  spark  to  occur  a  revolution  too  soon,  i.  e.,  just  as  the  fresh 
charge  is  entering  the  cylinder. 

5.  Misfiring. — This  is  said  to  take  place  when  the  charge  is  drawn 
into  the  cylinder  and  compressed,  but  passes  through  the  whole 
working  stroke  without  any  explosion  taking  place.     In  a  multi- 


84  AIR  SERVICE  HANDBOOK. 

cylinder  engine  the  unexploded  charge  as  it  leaves  the  cylinder  and 
comes  into  contact  with  the  hot  exhaust  gases  may  cause  an  explo- 
sion to  occur  in  the  muffler  or  exhaust  pipe.     Misfires  are  caused  by — 

A.  The  mixture  containing  too  much  or  too  little  gasoline,  thus 
forming  a  nonexplosive  charge.  Misfires  due  to  this  cause  usually 
occur  in  all  or  pairs  of  cylinders;  if  the  latter  the  prime  cause  will 
probably  be  traced  to  a  faulty  designed  intake  manifold  or  badly 
adjusted  carbureter.  Heating  the  intake  manifold  or  air  supply 
to  the  carbureter  may,  however,  overcome  it. 

B.  Poor  compression  due  to  causes  already  mentioned.  Misfiring 
in  this  case  occurs  in  one  or  more  cylinders  independent  of  the  rela- 
tive position  with  regard  to  the  inlet  pipe. 

C.  The  mixture  containing  a  quantity  of  exhaust  gas  and  so  being 
too  weak.  Cause,  a  leaky  exhaust  valve.  The  leakj^  exhaust  valve 
would  also  prevent  good  compression  so  that  this  really  comes  under 
the  same  heading  as  "B." 

D.  Defective  ignition.  With  modern  high-tension  magnetos 
defective  spark-plugs  are  the  most  common  cause  of  misfire. 

6.  Preignition. — This  happens  when  the  mixture  is  fired  on  the 
compression  stroke  (usually  without  the  aid  of  a  spark)  thereby 
tending  to  make  the  engine  run  backward.  This  is  often,  quite 
wrongly,  called  "back  firing."     This  may  be  caused  by — 

A.  The  ignition  spark  being  advanced  too  much  when  starting. 
Explosion  will  occur  before  crank  is  over  dead  center,  making  the 
engine  run  backward  a  few  revolutions.  This  should  always  be 
very  carefully  guarded  against  when  starting  an  engine  by  hand,  as 
the  wrench  given  to  the  starting  handle  when  preignition  occurs  is 
sufficient  to  sprain  or  even  break  the  operator's  wrist. 

B.  A  hot  piston  or  cylinder,  due  to  the  spark  being  too  far  retarded, 
causing  much  of  the  heat  of  the  explosion  to  pass  into  cylinder  walls 
and  piston  head.  This  causes  the  fresh  mixture  to  become  over- 
heated during  compression  and  so  to  explode  prematurely.  Pre- 
ignition, even  if  it  does  not  actually  force  the  engine  round  in  the 
wrong  direction,  may  cause  very  heavy  "knocking"  in  the  bearings, 
which  will  strain  the  engine  and  reduce  its  speed.  If  it  be  very 
excessive  it  may  easily  produce  fracture  of  some  part  of  the  mecha- 
nism, usually  the  crank  shaft. 

C.  Overheated  piston,  etc.,  due  to  too  weak  a  mixture.  This  is 
not  an  uncommon  result,  in  cold  weather,  of  too  economical  a 
carbureter. 

7.  Smoky  exhaust. — A  smoky  exhaust  may  be  caused  by  too  rich 
a  mixture  or  by  overlubrication;  the  excess  of  oil  supplied  to  the 
piston  and  cylinder  is  sucked  up  the  sides  of  the  piston  during  the 
suction  stroke  and  partially  burnt.     A  too  rich  mixture  will  usually 


AIR  SERVICE  HANDBOOK.  86 

leave  black  specks  on  one's  hand  if  it  is  ])ut  into  the  smoke  of  the 
exhaust. 

8.  Overheated  cylinders. — Haviiifj  the  mixture  too  weak.  i.  e.,  con- 
taining too  much  ail",  is  by  far  the  most  common  cause.  Too  much 
gasoline  will  also  overheat  the  cylinders,  thoufjh  to  nowhere  near  the 
same  extent  as  too  little  gasoline.  The  ignition  too  far  advanced  or 
too  far  retarded  is  also  a  cause  of  the  cylinders  running  hot. 

X.  IGNITION. 

There  are  two  types  of  ignition  arrangements: 

1.  Battery. 

2.  Magneto. 

The  first  system  has  been  largely  superseded  by  the  second,  but 
owing  to  the  fact  that  it  gives  practically  a  continuous  spark  without 
the  assistance  of  the  engine  it  is  retained  in  many  motor-car  engines 
to  facilitate  starting  M^hen  the  engine  is  cold.  The  magneto  is 
usually  run  off  the  engine  so  that  the  engine  must  have  a  certain 
speed  to  make  a  spark.  Sometimes  a  starting  magneto  turned  by 
hand  is  fitted  in  order  to  make  a  spark  while  the  engine  is  at  rest. 
Both  systems  are  eventually  dependent  on  the  same  principle,  which 
is  converting  a  low-power  electric  current  to  a  high-power  one  which 
is  strong  enough  to  jump  across  the  point  of  a  spark  plug  and  make  a 
spark. 

To  make  a  spark  there  must  be  a  generator  for  the  electric  current, 
a  circuit  to  carry  the  current,  and  a  gap  for  the  current  to  jump  across 
and  create  the  spark.  The  circuit  must  be  insulated  to  prevent  loss 
of  current. 

If  an  electric  current  passes  through  a  conductor  (i.  e.,  a  piece  of 
wii-e),  as  soon  as  the  current  commences  to  flow  a  magnetic  field  is 
created  around  that  conductor  from  which  lines  of  force  will  move  out 
radially.  The  magnetic  field  around  a  single  wire  would  not  be  very 
strong  so  that  the  conductor  is  wound  in  a  coil  or  many  coils  to  give 
the  requisite  strength.  The  coils  must  be  insulated  so  that  the  con- 
ductor of  one  coil  does  not  actually  touch  another.  If  this  conductor 
is  wound  round  a  core  of  soft  iron  the  magnetic  field  will  be  stronger 
still.  When  the  current  is  switched  on  lines  of  force  will  spring  out 
from  this  central  core  and  when  the  current  is  stopped  these  lines  of 
force  will  fall  back  again. 

If  lines  of  force  cut  a  conductor  or  if  a  conductor  is  moved  through 
a  magnetic  field,  a  current  of  electricity  is  produced  in  the  conductor 
and  this  is  called  an  "induced  current."  Supposing  we  wind  wire 
outside  the  coils  of  wire  we  have  referred  to  above,  making  sure  that 
the  wire  is  well  insulated,  every  time  the  lines  of  force  spring  out 
from  the  central  core  they  will  cut  the  outside  coils  of  wire  and  pro- 


86  AIR  SERVICE  HANDBOOK. 

duce  in  the  conductor  an  electric  current.  This  happens  eveiy  time 
we  either  make  or  break  the  connection  of  the  inner  circuit.  This 
inner  circuit  is  called  the  "primary  circuit"  and  the  outer  one  the 
"secondary  circuit."  It  should  be  noted  that  a  current  is  induced 
inthesecondary  circuit  only  at  the  moments  of  "make"  and  "break" 
because,  as  we  have  said,  the  current  induced  in  the  secondary 
depends  on  the  cutting  of  the  conductor  by  the  lines  of  force  and  this 
only  happens  when  they  spring  out  or  fall  back  toward  the  central 
core. 

So  long  as  lines  of  magnetic  force  are  cutting  the  conductor  it 
does  not  matter  if  the  magnet  or  conductor  are  moving;  in  either  case 
a  current  will  result. 

Magneto  ignition.— -The  fundamental  principle  on  which  the  mag- 
neto works  may  be  expressed  sinaply  as  that  in  which  a  closed  coil  of 
wire  rotates  within  the  field  of  force  of  a  magnet  and  cuts  through  the 
lines  of  force.    A  current  is  induced  twice  per  revolution  of  the  coil. 

^--   -  r  t7////////////////////////777Zok  '--^    5 

■^\V •^  -■"    -   -  -  -^^ -,  v\  -  .  ^ 
^  .  -  >  :r  -  -  :^  "-  '  / ' 

Fig.  30. 

The  field  of  force  exerted  by  a  magnet  is  easily  demonstrated  by 
placing  a  sheet  of  paper  over  the  poles  of  the  magnet  and  then 
sprinkling  iron  filings  on  the  paper.  The  filings  ^vill  take  up  clearly 
defined  positions  around  the  poles. 

Some  metals  retain  their  magnetism  permanently,  while  others 
lose  it  at  once.  An  example  of  the  former  is  hardened  steel,  and  of 
the  latter  soft  iron.    Advantage  is  taken  of  this  fact  in  the  magneto. 

The  magneto  consists  essentially  of  two  or  more  horseshoe-shaped 
magnets  placed  side  by  side  (in  some  magnetos  there  is  a  pair  of 
double  magnets  side  by  side  and  in  which  one  magnet  is  placed  on 
top  of  the  other).  The  ends  of  the  magnets  are  termed  "poles," 
i.  e.,  north  and  south.  Attached  to  the  poles  by  screws  are  pieces  of 
very  soft  cast  iron,  which  are  cut  away  into  semi-circular  form  inside 
the  horseshoe.  It  is  across  this  'polar  space"  that  the  magnetic 
lines  of  force  are  concentrated.  Within  the  horseshoe  and  the  semi- 
circular pole  pieces  an  "armature"  is  made  to  rotate.  This  "arma- 
ture" consists  of  a  shuttle-shaped  core  around  which  primary  and 
secondary  windings  are  coiled  exactly  in  the  same  manner  as  in  an 
induction  coil.    The  "armature"  is  made  up  of  a  number  of  soft 


AIR  SERVICE  HANDBOOK.  87 

iron  plates  in  order  that  it  may  obtain  and  lose  its  magnetism  very 
quickly. 

As  the  "armature"  rotates  in  the  magnetic  field  it  is  evident  that 
there  are  two  positions  in  each  revolution  when  the  coils  are  being 
cut  by  the  largest  number  of  lines  of  force. 

These  are  called  the  "'maximum  positions,"  and  it  is  at  these 
points  that  the  current  is  induced  in  the  primary  winding.  A  new 
field  of  force  is  then  created,  due  to  the  current  passing  through  the 
primary,  and  this  field  is  further  strengthened  by  the  core  of  the 
■'armature"  becoming  itself  a  magnet.  These  new  lines  of  force  cut 
the  secondary  winding  and  induce  a  current  in  that,  adding  still 
another  'field. "  The  current  of  the  primary  is  then  broken  at  the 
"contact  breaker"  and  the  field  belonging  to  the  primary  collapses, 
but  slowly  owing  to  the  influence  of  the  lines  of  force  of  the  secondary, 
the  current  still  tending  to  flow  in  the  same  direction.     At  this  point 


Fig.  31. 

the  "condenser"  comes  into  play  and  a  sudden  reversal  of  the  direc- 
tion of  the  current  in  the  primary  occurs.  So  rapidly  do  these 
motions  take  place  that  the  spark  occurs  at  the  plug  at  the  same 
instant  as  the  breaking  of  the  primary  circuit.  It  is  thus  seen  that 
two  sparks  are  obtained  every  revolution  of  the  "armature"  and  the 
speed  of  rotation  therefore  has  to  be  r^ulated  to  the  number  of 
cylinders  in  the  engine. 

Although  there  are  only  two  positions  in  which  the  maximum 
number  of  lines  of  force  cut  the  "armature"  windings,  yet  immedi- 
ately before  and  after  these  positions  are  reached  there  will  still  be 
enough  lines  of  force  cutting  the  primary  to  give  a  current.  It  is 
this  fact  which  allows  the  ignition  to  be  advanced  or  retarded  at  will 
by  altering  the  moment  when  the  current  in  the  primary  is  broken. 

The  primary  current  is  broken  mechanically  by  a  fiber  stop  on  the 
end  of  a  bell-crank  lever  carrying  one-half  of  the  "contact  breaker. " 
Rollers  are  fixed  in  the  circular  track  passed  through  by  the  fiber 
stop  in  its  revolution  and  as  the  top  passes  them  the  bell-crank 
lever  is  swung  about  its  fulcrum,  parting  the  two  screws  forming  the 
sides  of  the  "contact  breaker."     One  end  of  the  primary  circuit  is 


88 


AIR  SERVICE  HANDBOOK. 


"grounded"  to  the  "'armature"  core  and  the  other  connected  to  the 
fixed  half  of  the  "contact  breaker,  "  which  is  carried  on  the  "arma- 
ture" spindle.  The  secondary  circuit  is  usually  connected  to  one 
end  of  the  primary  so  as  to  be  "grounded."  The  other  is  connected 
to  a  slip  ring,  where  a  brush  collects  the  secondary  current  produced 
by  the  rupture  of  the  primary  current  and  passes  it  on  to  the  dis- 
tributor and  thence  to  the  spark  plugs.  The  distributor  is  on  the 
same  principle  as  that  described  above  for  "accumulator"  ignition. 
The  "condenser"  is  connected  in  parallel  with  the  two  sides  of 
the  "contact  breaker,"  i.  e.,  the  two  plates  are  connected  to  the 
two  parts  between  which  the  break  in  the  electrical  circuit  occurs. 

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Secondar(^  Circui  t-B/a 
Fig.  32. 

To  protect  the  insulation  of  the  "condenser"  from  being  pierced 
by  the  high  voltage  when  one  of  the  leads  to  the  plug  is  withdrawn 
a  "safety  spark  gap"  is  provided  near  the  brush  collector  on  the 
slip  ring.  This  acts  as  a  sort  of  "safety  valve, "  for  as  soon  as  the 
voltage  or  "electrical  pressure"  rises  too  high  a  spark  jumps  across 
this  spark  gap,  thus  relieving  the  electrical  pressure  in  the  circuit. 
To  stop  the  flow  of  current  to  the  plugs  in  order,  for  instance,  to  stop 
the  engine,  the  end  of  the  primary  winding  leading  to  the  "contact 
breaker"  is  connected  by  a  carbon  brush  to  a  switch.  By  "closing" 
this  switch  this  end  of  the  primary  winding  is  put  to  "earth"  and 
the  coil  is  thus  turned  into  a  closed  circuit.  Hence  no  "make" 
and  "break"  can  occur  in  the  primary  and  so  no  current  is  gener- 
ated in  the  secondary  circuit.  When,  however,  the  switch  is 
"opened"  the  "contact  breaker"  again  comes  into  action  and  the 


AIB  SERVICE  HANDBOOK.  89 

magneto  if  revolved  will  give  a  spark.  A  terminal  is  always  pro- 
vided for  this  purpose  on  the  magneto. 

There  is  another  kind  of  magneto  in  general  use  in  which  the 
"armature"  itself  does  not  revolve.  A  shuttle  of  soft  iron  is  revolved 
in  between  the  ''armature'"  and  the  pole  pieces.  This  shuttle 
distorts  the  magnetic  field  and  thus  draws  the  lines  of  force  across 
the  conductors  wound  round  the  "armature."  Maximum  current 
is  given  when  the  lines  of  force  are  springing  back  to  the  normal, 
and  this  occurs  four  times  during  a  revolution. 

Faults  m  ianitinn.  -The  faihire  of  electrical  arrangements  may  be 
due  to — 

1.  Battery  and  coil  ignition  (if  fitted). 

2.  The  magneto  ignition. 

3.  The  spark  plugs. 

If  dual  ignition  is  fitted — that  is,  if  there  are  t\\  o  coils  or  two  mag- 
netos— try  each  in  turn.  One  of  the  two  probably  will  be  foimd 
correct,  and  the  fault  thus  partly  located.  Should  both  fail,  remove 
the  spark  plugs  and  clean  them  with  gasoline,  replace,  and  try  again. 

When  testing  the  two  ignitions,  the  spark  plugs  can  be  short-cir- 
cuited to  the  cylinders  by  means  of  any  steel  tool  having  a  wooden 
handle  to  hold  it  by,  e.  g,  a  screw  driver.  If  a  spark  be  observed 
between  the  end  of  the  screw  driver  and  the  cylinder,  it  will  show 
that  the  high-tension  current  is  at  any  rate  reaching  this  point,  and 
the  length  of  the  spark  will  denote  its  intensity.  This  indicates 
that  the  ignition  is  producing  a  spark  and  hence  the  fault  must  be 
in  either — 

A.  The  plug  itself;  or 

B.  In  the  timing  of  the  magneto  or  its  electrical  coiniections,  or 
in  the  batteries  (if  fitted). 

To  test  for  "A,"  faulty  plugs,  the  plug  should  be  removed  and 
tested  by  passing  a  high-tension  current  through  it  in  air  imder 
about  100  poimds  pressure  per  square  inch.  If  a  good  spark  passes 
across  the  plug  terminals  and  no  signs  of  "flashing"  occur  else- 
where in  the  plug,  it  may  be  assumed  to  be  in  working  order.  It  is 
no  use  testing  the  plug  electrically  under  atmospheric  pressure,  as 
the  resistance  of  the  spark  gap  under  pressure  is  so  much  greater 
than  imder  atmospheric  conditions  that  a  Haw  in  the  insulation  which 
may  have  sufhcient  resistance  to  prevent  a  short  circuit  occiu'ring 
under  these  conditions  will  break  down  luider  th<»  moderate  i)ressures 
obtained  when  actually  working. 

With  rotating  cylinder  engines  the  terminal  at  the  end  of  the  plug 
is  pulled  outward  with  considerable  force.  In  plugs  in  which  mica 
is  used  as  the  insulation  there  is  a  danger  of  the  center  pole  of  the 
plug  l)ecoming  bent  toward  one  side  of  the  phig  and  thus  short- 


90  AIE   SERVICE   HANDBOOK. 

circuiting  the  current  when  the  engine  is  rotating.  Any  plug  in 
which  the  central  pole  is  bent  or  at  all  loose  should  be  at  once  changed 
or  adjusted. 

To  test  for  "B,"  timing  magneto  and  "distributor,"  if  the  fault  is 
elsewhere  than  in  the  plugs  test  the  timing  of  the  ignition  as  f ollow  s : 

Turn  the  engine  by  hand  slowly  and  note  the  timing  of  the  spark 
in  the  different  cylinders,  observe  the  crank  angle  (and  inlet  and 
exhaust  valves)  at  the  instant  when  the  two  sides  of  the  "contact 
breaker  "  come  apart.  This  indicates  that  a  spark  will  occur  at  this 
point  in  the  cycle.  If  this  is  correct,  the  various  ignition  leads  should 
then  be  traced  most  carefully  to  make  sure  that  they  are  connected 
up  to  the  right  cylinders  and  securely  fastened  to  their  respective 
terminals.  If  no  spark  appears  while  testing  the  terminal  of  the 
spark  plvig  by  short-cii'cuiting,  examine  all  the  electrical  connections 
and  see  that  none  of  them  have  come  adrift  or  have  been  connected 
up  to  the  wrong  terminals. 

A  common  cause  of  the  magneto  refusing  to  work  is  a  short  circuit 
to  "ground"  on  the  switch  connection.  This  prevents  the  primary 
current  being  broken  by  the  "contact  breaker"  and  consequently 
the  production  of  a  spark. 

The  platinum  points  on  the  "make"  and  "break"  may  want 
adjusting  or  the  coils,  condenser,  or  wire  leads  in  the  "armature" 
may  have  become  short- cii'cuited . 

When  machines  are  kept  in  tents,  especially  where  the  engine  is 
mounted  behind  the  magneto,  it  often  happens  that  the  magnetos 
become  damp  and  refuse  to  spark  properly.  This  may  be  prevented 
by  wrapping  up  the  magneto  at  night,  but  it  should  be  remembered 
that  damp  will  often  deposit  inside  a  covering  of  tliis  kind  after  the 
machine  has  been  up  in  the  cold  and  then  lands  and  is  put  into  the 
warm  tent. 

Defects  ivhich  may  occur  when  certain  specific  conditions  are  observed. — 

1.  A  fouled  plug:  To  find  out  which  cylinder  it  is,  slow  down  the 
engine  by  throttling  as  much  as  possible  and  then  short-circuit  each 
plug  in  turn  to  the  cylinder.  When  one  of  the  nonfaulty  plugs  is 
thus  shorted  the  engine  will  slow  down  considerably,  but  when  the 
foul  or  shorted  plug  is  treated  in  this  manner  no  difference  is  detected 
in  the  running  of  the  engine.  The  temperature  of  the  cylinders  wall 
often  indicate  the  defective  plug.     Replace  the  plug  mth  a  new  one. 

2.  Faulty  distributor:  Examine  the  carbon  brush  on  the  dis- 
tributor and  see  that  no  oil  has  got  to  it.  If  there  are  signs  of  grease, 
clean  it  off  with  gasoline  before  replacing  it.  The  carbon  brush  may 
have  worn  a  groove  round  the  distributor  and  the  metal  strips  lead- 
ing to  the  plug  terminals  may  have  got  masked  by  the  insulation. 
The  best  remedy  for  this  is  to  turn  up  the  inside  of  the  cylinder 
carrying  the  distributor  segments  in  a  lathe. 


AIR  SERVICE   HANDBOOK.  91 

3.  Defective  insulation  on  connecting  wire  to  plus?:  If  the  w-ire 
carries  a  high-tension  current  a  spark  will  probably  be  seen  at  the 
point  where  the  insulation  has  given  way.  Indications  will  be  the 
same  as  in  "1."  Examine  the  insulations  carefully  and  replace  the 
wire  if  necessary  by  a  new  length.  Should  the  flaw  in  the  insula- 
tion be  small,  a  repair  can  be  made  with  India-rubber  solution  and 
sticky  tape.  With  stranded  wire  care  should  be  taken  to  see  that 
all  strands  are  neatly  housed  in  the  terminal.  It  sometimes  happens 
that  one  strand  escapes  and  is  short-circuited  by  the  vibration  of 
the  engine,  which  causes  intermittent  missing  and  is  sometimes  hard 
to  detect. 

4.  Faulty  condenser:  Tliis  ought  not  to  occur  in  magnetos  where 
a  safety  gap  is  pr()\-ided  to  prevent  too  high  a  voltage  being  gener- 
ated in  the  secondary  circuit.  It  is  usually  indicated  by  sparking 
at  the  platinum  points  of  the  "contact  breaker."  Should  the  plat- 
inum points  of  The  "contact  breaker"  be  worn  and  pitted  the  same 
indications  will  be  present  so  that  it  is  as  well  to  examine  these 
platinum  points  first  of  all,  and  then  if  necessary  to  file  them  square 
and  smooth,  afterwards  adjusting  them  to  the  correct  distance  apart 
at  break— 0.4  millimeter  (16/1000  inchi. 

5.  If  all  the  cylinders  fire  weakly  on  the  magneto  circuit,  examine 
the  "contact  breaker"  and  its  adjustments. 

6.  No  spark  obtainable  with  the  magneto  circuit:  See  that  the 
wire  from  the  long-contact  terminal  to  which  the  switch  circuit  is 
connected  is  not  short-circuiting  to  the  frame.  The  ground  brush 
at  the  back  of  the  rocking  lever  of  the  magneto  "contact  breaker" 
may  be  oily  and  so  preventing  the  magneto  secondary  circuit  from 
being  completed . 

7.  No  sparking  at  any  terminal:  A  terminal  of  the  distributor  cir- 
cuit has  probably  come  loose  or  the  wire  short-circuited.  Examine 
both  carefully.  The  switch  contacts  should  also  be  examined  in  all 
these  cases. 

8.  The  magneto  refusing  to  stop,  producing  secondary  current  when 
switched  off:  This  is  probably  due  to  oil  having  got  underneath  the 
carbon  brush  on  the  short-circuiting  terminal  at  the  end  of  the  long- 
con  tact-^  screw  of  the  magneto  "contact  breaker."  Remove  the 
cover  over  this  latter  and  clean  the  end  of  the  brush  and  the  face 
it  bears  on  with  gasoline,  then  replace. 

XI.  MOTOR  TRANSPORT. 

Engines. — In  general  motor-car  engines  are  governed  by  the  same 
principles  as  those  applicable  to  air  engines.  The  general  chapter 
on  engines  must  therefore  be  read  in  conjunction  with  this  chapter. 
In  addition  the  following  points  should  be  noticed: 


92  AIR  SERVICE  HANDBOOK. 

A.  Bearings  should  normally  be  examined  after  10,000  miles. 
They  may  only  require  to  be  tightened  up  or  they  may  be  badly 
worn,  thus  necessitating  remetaling. 

B.  New  piston  rings  will  require  to  be  fitted  at  this  period. 

C.  Whenever,  for  any  reason,  an  engine  is  taken  down  it  is  ad- 
visible  at  the  same  time  to  grind  in  the  valves,  clean  the  pistons 
and  cylinder  heads,  and  clean  all  oil  leads  and  filters.  When  re- 
assembling the  engine  it  is  necessary  to  use  new  washers  and  packing 
throughout. 

D.  As  all  motor  cars  are  fitted  with  variable  ignition,  care  must  be 
taken  in  tuning  to  allow  sufficient  "advance"  and  "retard"  to  be 
given  on  the  ignition  quadrant.  In  cases  where  independent  mag- 
neto and  battery  ignition  are  fitted,  each  system  must  be  adjusted 
so  as  to  spark  at  the  same  point  in  the  cycle. 

Routine  examination. — Periodical  inspections  of  the  car  or  truck 
must  be  made  and  the  following  points  seen  to: 
Every  time  the  car  is  used — 

A.  Tires  correctly  inflated  and  spare  wheels  in  place,  and  tools  for 
changing  wheel  or  rims  (jack  and  brace). 

B.  Radiator  and  gasoline  tank  full.  Carry  spare  can  of  gasoline 
and  strainer. 

C.  Sufficient  lubricating  oil  in  the  pump  or  reservoir  and  that  the 
feeds  work  freely. 

D.  Batteries  properly  charged  and  coil  working. 

E.  Brakes  working  properly. 
Every  day  before  duty — 

A.  All  the  above  points. 

B.  Oil  holes  on  steering  arms,  knuckles,  universal  joint,  etc., 
cleaned  and  oiled.  This  should  be  done  after  the  car  has  been 
cleaned. 

C.  Grease  cups  on  springs  and  shackles  screwed  down  and  properly 
supplied  with  grease. 

Weekly— 

A.  Spark  plugs  and  ignition  looked  over,  magneto  oiled  and 
cleaned. 

B.  Examine  water  joints,  see  pump  packing  does  not  leak,  and 
also  that  the  radiator  is  tight. 

C.  Refill  axle  caps  and  examine  clutch  leather. 

D.  Open  gear  box  and  see  that  there  is  sufficient  grease. 

E.  Changing  tires  about  on  wheels  if  uneven  wear  is  noticed. 

F.  Examine  body  work. 
Monthly — 

A.  Grind  in  valves  (or  after  1,000  miles  running). 


AIR  SERVICE  HANDBOOK.  93 

Cure  of  grease  and  oil  caps.  -There  are  several  parts  on  the  ear 
which  require  regular  lubrication  and  which  are  not  supplied  auto- 
matically. These  parts  are  generally  equipped  either  with  grease 
cups  or  oil  holes.  Particular  note  should  be  made  of  oilers  on  the 
spring  hangers,  universal  joints,  steering  pivots,  knuckles,  steering- 
gear  box,  and  such  like.  It  is  also  necessary  periodically  to  intro- 
duce some  lubricant  between  the  lamiiiationti  of  the  springs. 

Drivers  of  motor  vehicles  should  make  themselves  thoroughly 
acquainted  with  all  the  grease  and  oil  cups  on  their  car.'<,  and  must 
systematically  keep  them  supplied.  The  frequency  of  the  applica- 
tion of  oil  or  grease  will  depend  on  the  amount  of  running. 

Care  of  clutch. — The  clutch  may  want  a  little  attention.  If  a 
leather  clutch  is  fitted  and  the  leather  comb  can  be  got  at,  it  may  be 
brushed  over  with  at  least  one  coat  of  castor  oil,  the  latter  being 
allowed  to  soak  in.  This  should  be  done  when  the  clutch  is  "fierce" 
owing  to  the  leather  becoming  glazed  and  hard.  It  does  not  follow 
that  a  new  clutch  leather  is  necessary  when  a  clutch  is  not  giving 
satisfaction.  Sometimes  it  will  be  found  that  a  shoulder  of  about 
one-sixteenth  inch  deep  has  worn  on  the  old  leather.  This  should 
be  carefully  trimmed  off  with  a  sharp  file,  which  mil  give  the 
leather  a  new  life.  This  allows  the  comb  to  go  farther  home,  giving 
a  closer  contact  between  the  surfaces. 

Especial  detail  to  watch  is  to  see  that  the  copper  rivets  are  well 
below  the  surface  of  the  leather.  If  they  become  flush  with  the 
leather,  the  result  would  be  a  nasty  gripping  or  even  difficulty  in 
disengaging.  A  metal  to  metal  clutch  requires  to  be  cleaned  out 
occasionally  with  kerosene.  A  hole  is  generally  provided  for  the 
purpose  in  the  clutch  casing.  If  the  clutch  takes  hold  with  a  jerk, 
a  little  thin  mineral  oil  will  put  it  right. 

Gears. — The  gear  box  should  be  regularly  inspected  to  see  that 
there  is  an  ample  supply  of  lubricant,  but  not  an  excess.  It  is  quite 
unnecessary  to  fill  up  the  cases,  as  this  will  only  result  in  the  gear 
grease  flooding  out  at  the  joints  and  bearings  and  making  them  a 
receptacle  for  mud  and  dust.  The  amount  of  lubricant  used  should 
be  sufficient  to  cover  the  lower  teeth  of  the  gears;  the  rest  will  look 
after  itself  (see  differential  gears). 

Differential  gear  and  chains. — The  differential  gear  transmits  the 
power  from  the  speed-change  gear  to  the  rear  axle  of  the  car.  Cars 
which  are  made  with  chain  drive  to  both  wheels  have  the  differ- 
ential gear  arranged  on  the  countershaft  at  the  ends  of  which  the 
chain  sprockets  are  fitted.  Usually  she  differential  and  chain-speed 
gear  are  fitted  in  the  same  case. 

Chains  require  to  be  renewed  occasionally  and  taken  up  as  they 
wear.  Clean  with  kerosene  and  lubricate  with  graphite  on  a  brush. 
Links  of  various  lengths  can  be  added. 


94  AIR  SERVICE  HANDBOOK. 

Care  of  brakes. — Attention  to  the  brakes  is  very  important.  They 
should  be  adjusted  as  closely  as  is  permissible,  the  jaws  being  set 
so  as  just  to  clear  the  drums  but  not  to  set  up  any  permanent  friction. 
A  screw  adjustment  is  provided  for  this  purpose.  Too  much  clear- 
ance lessens  the  responsiveness  of  the  brakes,  especially  in  an 
emergency.  The  rods  actuating  the  brakes  should  be  carefully 
examined  from  time  to  time  for  any  signs  of  weakness.  Particular 
attention  should  be  paid  to  insure  that  the  jointing  pins  have  split 
pins  properly  fitted. 

Cleaning  and  washing  cars. — The  car  ought  to  be  washed  down  as 
soon  as  it  comes  in,  without  giving  the  mud  a  chance  to  set.  On  no 
account  should  dust,  dirt,  or  mud  be  brushed  off.  It  must,  in  the 
fullest  sense  of  the  term,  be  washed  off  or  else  the  paintwork  will  be 
ruined.  If  a  hose  is  available,  it  will  be  very  useful  in  getting  the 
mud  off  the  under  parts  of  the  car  and  will  save  a  lot  of  time  and 
labor. 

In  using  the  hose  for  the  outside  of  the  car  (that  is,  for  the  wheels 
and  body  work  in  general)  the  following  points  should  be  observed : 

A.  Care  must  be  taken  that  the  water  does  not  go  anywhere  but 
where  it  is  intended  to  go.  It  should  not  be  splashed  about  in 
every  direction. 

B.  A  strong  pressure  of  water  from  the  nozzle  is  of  considerable 
advantage  in  cleaning  the  underparts  of  the  car,  where  the  mud 
is  generally  heaviest,  and  in  cleaning  the  underside  of  the  mud 
guards. 

C.  When  dealing  with  the  paintwork,  however,  a  strong  pressure 
of  water  is  quite  likely  in  removing  the  gritty  particles  at  the  same 
time  to  force  them  over  the  paintwork  and  scratch  it.  Apply  the 
water  with  little  force,  but  in  plenty.  If  this  is  done  when  the  car 
comes  in  wet,  the  mud  will  be  speedily  and  easily  removed.  If  the 
mud  has  been  allowed  to  dry,  the  water  must  be  poured  over  it,  so 
as  to  soften  it  first  of  all.  Afterwards  it  will  gradually  be  carried 
away  as  the  water  runs  over  it.  On  no  account  should  the  mud 
be  rubbed  off.  Brushing  or  rubbing  the  mud  off,  even  if  it  is  wet, 
will  cause  scratching  and  deterioration  of  the  paintwork. 

D.  When  all  the  dirt  and  mud  has  been  soaked  off,  the  surface 
can  be  gone  over  with  a  wet  sponge,  using  clean  water. 

E.  Oils  and  grease  are  bad  for  the  paintwork,  and  care  must  be 
taken  that  neither  gasoline,  kerosene,  or  lubricating  oil  is  allowed 
to  remain  on  any  part  of  the  paintwork. 

F.  When  dealing  with  a  car  which  is  soiled  with  dust,  the  same 
care  must  be  used  in  attempting  to  rub  it  off,  the  surface  should 
be  gone  over  first  with  a  full  sponge  and  finished  off  as  before. 


AIR   SERVICE   HANDBOOK.  96 

Care  of  tires. — If  the  foUowiiif^  points  are  attended  to  the  life  of 
tires  can  be  increased  considerably: 

A.  Cuts,  even  surface  cuts,  require  \Tilcanizing.  This  keeps  out 
the  water. 

B.  Tires  must  be  kept  up  to  pressure,  20  pounds  to  each  inch 
cross  section,  i.e.,  36  X4=80  pounds. 

C.  If  possible,  keep  two  spare  wheels,  so  that  repairs  can  be  carried 
out  on  one  while  the  other  is  ready  for  duty. 

D.  Watch  wheels  for  alignment.  If  a  tire  shows  abnormal  wear, 
look  to  the  axles  or  distance  rods. 

E.  Do  not  apply  brakes  abruptly,  except  in  emergency.  A  rapid 
"pull  up"  takes  a  good  deal  of  mileage  off  a  tire. 

XII.  INSTRUMENTS. 

The  barometer. — The  mercurial  barometer  is  the  standard  instru- 
ment for  measuring  the  pressure  of  the  atmosphere.  In  this  instru- 
ment the  pressure  of  the  atmosphere  is  compared  with  the  pressure 
at  the  base  of  a  column  of  mercury  of  known  height. 

If  a  tube  from  which  the  air  has  been  exhausted  is  placed  with  its 
open  end  in  a  small  cistern  of  mercurj',  the  pressure  of  the  atmosphere 
will  force  the  mercury  up  the  tube  until  the  pressure  at  the  level 
of  the  surface  of  mercury  in  the  cistern,  due  to  the  column  of  mercury 
in  the  tube,  is  equal  to  that  of  the  atmosphere  acting  downwards  on 
the  surface  of  the  mercury  in  the  cistern.  The  pressure  of  the 
atmosphere  is  conveniently  given  in  terms  of  the  length  of  this 
column  of  mercur^^ 

An  actual  barometer  consists  essentially  of  the  exhausted  tube 
dipping  into  a  cistern  of  mercury  as  detailed  above.  Alongside 
the  glass  tube  is  fixed  a  scale  over  which  moves  a  vernier.  The 
vernier  is  set  exactly  level  with  the  top  of  the  mercury. 

As  the  mercury  rises  in  the  exhausted  tube  the  level  of  the  mercury 
in  the  cistern  will  fall,  so  that  if  the  scale  be  fixed  its  readings  will 
no  longer  give  the  true  distance  between  the  surface  of  the  mercury 
in  the  tube  and  of  that  in  the  cistern.  To  eliminate  this  error  one 
of  two  methods  may  be  adopted.  In  the  Fontin  barometer  the 
bottom  of  the  cistern  is  made  of  wash  leather  and  can  be  raised  or 
lowered  by  means  of  a  screw  until  the  surface  of  the  mercurj^  always 
just  touches  a  fixed  mark.  In  the  Kew  pattern  barometer  the 
length  of  the  divisions  of  the  scale  on  the  tube  is  slightly  altered  so 
that  it  always  reads  the  correct  height  without  adjusting  the  mer- 
cury in  the  cisteni. 

Errors  and  their  correction. — 

A.  Temperature:  The  first  thing  to  be  allowed  for  is  the  tem- 
perature of  the  barometer.  If  the  temperature  rises  it  affects  the 
barometer  in  two  ways — 


96 


AIB  SERVICE  HANDBOOK. 


1.  The  mercury  expands  and  therefore  rises  in  the  tube.  This  is 
equivalent  to  an  apparent  increase  of  pressure. 

2.  The  scale  against  which  the  height  of  the  mercury  is  measured 
expands,  causing  an  apparent  decrease  of  the  height  of  the  mercury 
or  a  fall  of  pressure. 

To  eliminate  the  effects  of  change  of  temperature  the  readings  of 
the  barometer  are  always  corrected  to  what  they  would  be  if  the 
whole  barometer  were  at  32°  F.  This  correction  depends  on  the 
actual  temperature  at  the  time,  the  coefficient  of  expansion  of 
mercury,  and  that  of  the  scale. 

n 

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F.Hid    r'o.'AL- 


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Fig.  33. 

B.  Height:  With  a  view  to  comparing  the  pressures  at  two  or 
more  stations,  as,  for  example,  in  order  to  construct  a  daily  weather 
map,  allowance  must  be  made  for  the  different  heights  of  the  sta- 
tions. The  higher  the  station  the  less  will  be  the  pressure.  For 
purposes  of  comparison,  therefore,  the  reading  of  the  barometer  is 
always  corrected  to  what  it  would  be  if  the  station  were  at  sea  level. 
The  amount  to  be  added  is  given  by  the  formula: 

log  jB- log  6=yy|^g[n-0.00204(i-32)] 

where  B  is  the  pressure  at  sea  level,  b  the  pressure  at  the  station, 
h  height  of  the  station  in  feet  above  sea  level,  t  temperature  of  the 
air  in  degrees  Fahrenheit. 


AIR  SERVICE  HANDBOOK. 


97 


For  convenience  tables  giving  this  correction  can  be  constructed 
but  a  special  table  must  be  made  for  each  station. 

As  far  as  an  aviator  is  concerned  a  rough  rule  for  height  within  a 
few  thousand  feet  of  the  ground  is  as  follows:  The  column  of  mercury 
falls  1  inch  for  every  1,000  feet  of  height.  > 

C.  Index  errors:  The  scale  may  not  indicate  the  true  distance 
from  the  surface  of  the  mercury  in  the  cistern. 

D.  Scale  errors:  The  graduations  of  the  scale  may  not  be  tlie  right 
distance  apart. 

These  two  errors  'C*  and  "W  are  best  found  by  having  the 
instrument  tested  against  a  standard  barometer  and  corrections 
must  be  applied  to  allow  for  them. 

E.  Imperfect  vacuum:  If  a  small  ([uantity  of  aii'  be  left  in  the 
tube  aboA-e  the  mercurv  it  will  cause  the  reading  to  be  too  low.  the 


'Po.udr 


K///////////////:'^^^//////x/////.<//^^^^ 


Fig.  34. 

amount  varying  with  the  temperature  and  height  of  the  barometer 
This  can  easily  be  tested  by  tilting  the  barometer  gently  until  the 
mercury  reaches  the  top  of  the  tube.  If  it  hits  the  top  with  a  sharp 
click  the  vacuum  may  be  considered  good. 

Aneroids  and  hnrfxjraphs. — The  mercury  barometer  is  the  only 
eally  reliable  instrument  for  measuring  the  atmospheric  pressure 
acurately.  It  is.  however,  not  a  portable  instrument  and  for  many 
purpo.«es  an  aneroid  is  more  fonvenient.  This  instrument  contains 
one  or  more  flexible  metal  boxes  from  which  the  air  has  been  partially 
exhausted.  The  pressure  of  the  atmosphere  on  the  outside  is  always 
tending  to  compress  the  sides  of  the  box  while'a  spring  placed  inside 
tends  to  push  the  sides  out.  Hence,  as  the  atmospheric  pressure 
changes  the  box  will  be  expanded  or  compressed.  By  means  of 
suitable  levers  the  motion  of  the  box  causes  a  pointer  to  move  over 
a  graduated  scale.  This  scale  is  generally  graduated  so  that  it  gives 
pressures  in  terms  of  inches  of  mercury  as  measured  on  a  mercurial 
4664:3 — 18 7 


98  AIR  SERVICE  HANDBOOK. 

barometer.     For  air  work  it  is  usually  graduated  in  hundreds  and 
thousands  of  feet. 

Errors:  It  should  be  noticed  that  both  the  original  setting  of  the 
instrument  as  taken  by  the  makers  from  the  standard  barometer  and 
the  scale  value  depend  on  the  adjustment  of  the  instrument.  The 
scale  value,  i.  e.,  the  size  of  the  graduations  of  the  scale,  is  generally 
accurate,  but  the  absolute  value  of  the  pressure  in  inches  of  mercury 
given  by  an  aneroid  can  seldom  be  relied  on.  This  latter  value  will 
vary  if  either  the  scale  or  the  pointer  be  permanently  moved  rela- 
tively to  each  other  (e.  g.,  by  altering  the  adjusting  screw  or  moving 
the  needle).  Therefore,  the  aneroid  is  used,  it  is  for  reading  differ- 
ences of  pressure  and  not  the  absolute  pressure  at  any  given  moment, 
unless  they  have  been  calibrated  with  reference  to  a  standard 
mercury  barometer. 

Aneroids  are  fi-equently  subject,  also,  to  errors  due  to  changes  of 
temperature.  As  the  temperature  rises  the  elasticity  of  the  spring 
inside  the  flexible  boxes  decreases  and  the  boxes  are  compressed 
more  than  they  would  otherwise  be.  This  causes  an  apparent  rise 
of  pressure.  Even  many  so-called  "compensated  "  aneroids  are  sub- 
ject to  some  error  from  this  cause. 

Any  form  of  barometer  which  gives  a  written  record  of  the  changes 
of  pressure  is  known  as  a  barograph.  These  are  generally  of  the 
aneroid  type.  Instead  of  the  motion  of  the  aneroid  boxes  being 
communicated  to  a  simple  pointer  moving  over  a  scale,  it  is  communi- 
cated to  a  pen  moving  over  a  chart  Avound  round  a  revolving  drum 
driven  by  clockwork.     Otherwise  the  principle  is  the  same. 

Such  barographs  are  subject  to  the  usual  errors  of  aneroids,  and  in 
addition  errors  may  be  introduced  by  excessive  friction,  either  in 
the  levers  or  between  the  pen  and  paper.  If  as  the  barometer  rises 
or  falls  the  pen  moves  up  or  down  in  a  sudden  step  there  is  almost 
certainly  friction.  Variation  of  pressure  is  almost  always  gradual. 
The  friction  may  frequently  be  minimized  by  reducing  the  pressure 
of  the  pen  on  the  paper;  but  sometimes  the  pivots  of  the  bearings 
need  adjustment.  In  this  latter  case  the  instrument  should  be  sent 
back  to  the  makers. 

Aneroids  and  barographs  are  frequently  used  to  measure  difference 
of  heights.  This  is  done  by  measuring  the  difference  of  atmospheric 
pressure  at  the  two  places  to  be  compared.  The  difference  of  pres- 
sure is  due  to  the  difference  of  the  weight  of  air  above  the  levels  of 
the  two  places.  The  actual  difference  of  pressure  corresponding  to 
any  difference  of  height  is  given  by  the  formula  above.  From  this 
it  is  seen  that  the  difference  of  pressure  depends  to  some  extent 
upon  the  temperature  of  the  air  and  upon  the  mean  pressure. 

On  aneroids  and  barographs  which  are  provided  with  a  scale  show- 
ing heights  it  is  necessajry  when  fixing  the  scale  to  decide  upon  a 


AIR   SERVICE   HANDBOOK.  99 

mean  pressure  and  temperature  of  the  air.  W'lien  the  pressure  and 
temperature  of  the  air  differ  from  the  assumed  mean  it  is  obvioua 
that  the  height  reading  obtained  will  not  be  quite  correct.  This 
error  may  amount  to  about  4  per  cent  on  account  of  difference  of 
temi)erature  and  to  about  4  per  cent  on  account  of  the  difference 
from  mean  pressm-e.  Some  aneroids,  however,  are  now  made  in 
i?uch  a  way  that  the  errors  due  to  the  pressure  being  different  from 
the  mean  are  very  greatly  reduced. 

Aneroids  in  an  airplane  are  the  only  means  of  obtaining  the  height 
above  the  ground.  The  scale  is  set  at  zero  before  the  machine 
leaves  the  ground  and  the  heights  shown  by  the  aneroid  are  those 
above  the  level  of  the  airdrome.  This  should  be  borne  in  mind 
when  bomb  dropping  because  the  target  may  be  very  much  higher 
or  very  much  lower  than  the  airdrome  and  this  will  make  a  large 
error  in  the  fall  of  the  bomb.  Then  again,  for  bomb  dropping  at 
great  distances  the  pressure  of  the  air  over  the  target  may  be  very 
different  from  the  pressure  above  the  airdrome  when  the  machine 
started. 

There  is  usually  a  great  lag  in  the  instrument  which  makes  the 
readings  too  low  when  ascending  and  too  high  when  descending. 
For  this  reason  one  can  never  use  an  aneroid  to  tell  the  height  at 
which  one  should  straighten  up  to  land  a  machine.  This  should  be 
borne  in  mind  if  one  has  to  make  a  landing  in  the  fog  or  in  darkness. 
When  the  aneroid  shows  zero  on  account  of  a  lag  the  machine  is 
probably  a  little  below  and  may  strike  the  ground  at  any  moment. 

Anemometers,  or  air-speed  indicators. — -The  anemometer  in  general 
use  up  to  about  15  years  ago  was  that  known  as  the  Robinson  anemom- 
eter. This  instrument  consisted  of  four  arms  which  were  capable 
of  turning  about  a  vertical  axis.  Each  arm  carried  a  hemispherical 
cup  at  its  end.  In  consequence  of  the  wind  having  more  force  on 
the  concave  side  of  the  cups  than  on  the  convex  side  these  cups  were 
driven  round  with  a  speed  proportional  to  the  velocity  of  the  wind. 
A  counting  gear  was  attached  to  show  the  number  of  times  the  cups 
turned  round.  This  instrument  will  only  give  satisfactorily  the 
mean  wind  over  a  given  time.  For  most  aviation  purposes  the 
gustiness  of  the  wind  or  when  flying  the  sudden  changes  in  speed  is 
perhaps  the  more  important  matter.  The  Robinson  anemometer 
gives  no  indication  of  this  and  is  therefore  of  little  use  for  aeronautical 
purposes. 

A  much  more  useful  instrument  is  that  designed  by  Dines.  In 
this  anemometer  a  wind  vane  is  mounted  so  as  to  turn  round  freely 
with  the  wind.  The  front  part  of  the  vane  is  in  the  form  of  a  tube, 
the  opening  of  which  is  always  kept  facing  the  wind.  This  is  the 
pressure  opening.  The  hollow  vane  is  connected  through  an  air- 
tight joint  to  a  pipe  leading  to  the  recording  apparatus  below.     The 


100  AIR  SERVICE  HANDBOOK. 

wind  blowing  down  the  vane  increases  the  pressure  inside  the  tube. 
A  second  pipe  led  from  the  recording  apparatus  opens  just  below 
the  vane  in  a  series  of  small  holes  arranged  symmetrically  round  a 
vertical  tube.  As  the  wind  blows  past  these  holes  it  produces  a 
small  suction. 

The  two  tubes  are  connected  to  some  form  of  pressure  gauge  which 
indicates  the  velocity  of  the  wind.  The  gauge  in  the  self-recording 
instruments  consists  of  a  vessel  floating  in  water.  The  inside  of  the 
float  is  connected  to  the  pressure  tube  from  the  vane  and  the  space 
between  the  outside  of  the  float  and  the  containing  vessel  is  connected 
to  the  suction  tube.  The  principle  is  the  same  as  that  of  a  gasometer, 
assuming  that  the  latter  were  inclosed  in  a  case.  As  the  difference 
of  pressure  between  the  air  in  the  two  pipes  increases  the  float  is 
raised  out  of  the  water.  A  pen  attached  to  the  float  records  the  veloc- 
ity at  each  instant  on  a  chart  rolled  round  a  drum  driven  by  clock- 
work. The  pressiu*e  produced  by  the  wind  is  proportional  to  the 
square  of  the  velocity  so  that  the  float  must  be  made  of  a  special 
shape  if  its  movements  are  to  be  proportional  to  the  square  root  of 
the  pressure,  i.  e.,  equal  to  the  velocity  and  not  to  the  pressure, 
which  would  be  the  case  if  the  float  were  cylindrical. 

This  instrument  requires  very  little  attention.  In  setting  up  its 
indication  should  be  compared  with  some  form  of  pressure  gauge  to 
see  that  the  readings  are  correct. 

The  density  of  the  water  varies  with  the  temperature  and  this 
causes  the  zero  of  the  instrument  to  change.  To  correct  this  the 
float  is  weighted  with  a  few  shot  so  that  the  zero  is  always  correct. 
It  is  also  important  to  see  that  the  level  of  the  water  is  kept  constant 
at  the  fixed  mark  (the  float  being  in  the  zero  position  when  this  is 
tested). 

The  pressure  produced  by  the  wind  is  proportional  to  the  density 
of  the  air.  Any  cause  which  makes  the  density  of  the  air  change 
will  therefore  alter  the  indicated  velocity.  The  density  of  the  air 
is  changed  by  variations  of  temperature  and  pressure.  But  the 
changes  produced  at  the  earth's  surface  from  these  causes  are  too 
small  to  be  important  for  matters  connected  with  aviation.  WTien, 
however,  instruments  on  the  same  principle,  e.  g.,  Pitot  tubes  and 
air-speed  indicators  are  taken  up  in  airplanes  to  show  the  velocity 
of  travel  through  the  air  the  change  produced  in  the  readings  by  the 
decrease  of  pressure,  and  therefore  the  density  of  the  air,  may  be 
appreciable.  Thus,  at  5,000  feet  the  indicated  speed  will  be  7  per 
cent  below  the  true  speed.  In  addition  to  recording  the  velocity 
of  the  wind  the  Dines  instrument  may  also  be  fitted  to  show  the 
direction.  In  this  case  a  rather  larger  vane  is  provided  than  is 
usual  with  the  smaller  instrument  and  it  is  mounted  on  ball  bearings. 
The  vane  is  connected  to  a  rod  which  passes  vertically  down  to  the 


AIR  SERVICE  HANDBOOK. 


101 


recording  apparatus.  This  consists  of  a  drum  with  two  spirals 
which  engage  two  arms  connected  to  the  recording  pens.  As  the 
vane  turns  round  the  drum  is  also  turned  and  moves  the  pens  on 
the  paper.  This  type  of  anemometer  is  generally  designed  so  that 
it  records  both  the  velocity  and  direction  of  the  wnid  on  the  same 
chart. 

Air-speed  indicators  on  machines. — In  all  modern  machines  the 
indicator  usually  consist-s  of  a  pressure  head  mounted  to  point 
straight  ahead  on  one  of  the  outer  struts  of  the  machine,  and  this  is 
connected  by  aluminum  tubing  to  the  gauge  on  the  instrument 
board  of  the  machine.  This  head  is  very  much  the  same  as  that 
of  the  Dines  tube.  One  tube  points  straight  to  the  front,  the  other 
tube  is  arranged  to  have  no  pressure  in  it.  The  first  is  the  ' '  Pressure  " 
tube  and  the  second  the  "Static"  tube.  The  tubes  are  arranged 
this  way  to  insure  that  there  will  be  no  unknown  pressure  acting 
against  the  pressure  of  the  air  as  would  be  the  case  if  the  tube  opened 


Fig.  35.— PRESSURE  HEADS. 
P=  Pressure  tube.  S=Static  tube. 

just  anywhere  in  the  machine.  The  gauge  consists  usually  of  a 
diaphragm  of  some  material  which  can  be  moved  backward  and  for- 
ward as  the  pressure  increases  or  decreases.  The  center  of  the 
diaphragm  is  attached  by  a  silk  cord  or  by  gears  to  a  pointer  and 
this  pointer  indicates  the  pressure  of  the  air  at  the  pressure  opening. 
The  readings  of  the  gauge  can  be  checked  by  connecting  the 
pressure  opening  to  a  U  tube  containing  liquid.  If  a  pressure  be 
applied  so  as  to  work  equally  on  both  the  gauge  and  the  liquid,  the 
liquid  will  rise  to  a  certain  height  and  the  gauge  will  show  a  certain 
reading.    The  air  speed  is  shown  by  the  following  formula: 

V   r 

where  S  equals  the  speed  in  miles  per  hour,  r  equals  the  density  of 
the  air,  k  equals  a  constant  depending  upon  the  shape  of  the  pressure 
head,  h  equals  the  height  of  the  liquid  in  inches  (water  at  normal 
density). 

From  this  it  is  seen  that  the  gauge  must  be  graduated  to  suit  the 
head;  that  is,  a  "mark  3"  head  should  not  be  used  with  a  "mark  4" 
gauge.  Also,  at  a  height,  the  readings  will  be  far  lower  than  the 
actual  speed. 


102 


AIR   SERVICE  HANDBOOK. 


P'c. 


Jt-cUi 


These  instruments  have  also  a  certain  lag  so  that  they  do  not  show 
small  quick  changes  in  speed.  A  pilot  can  stall  his  machine  and 
regain  flying  speed  without  the  instrument  ever  showing  that  he 
has  lost  his  flying  speed.  These  instruments  also  may  be  set  to  a 
wrong  zero  so  that  a  pilot  should  never  fly  by  his  air-speed  indicator 
only.  These  indicators  simply  show  large  mean  changes  in  the 
speed  of  the  airplane  and  are  best  used  as  a  guide  only. 

The  pipe  connecting  the  pressure  head  with  the  gauge  must  be  air- 
tight. The  joints  are  made  with  rubber  tubing  which  frequently  rots . 
Some  instruments,  such  as  the  Pitot  tube  itself,  depend  on  a 
height  of  liquid  or  on  unbalanced  parts  in  the  gauge.  These 
instruments  are  only  accurate  when  flying  in  a  straight  line.  If 
turns  be  made  the  unbalanced  portion  of  the  instrument  is  pulled 
down  by  centrifugal  force  so  that  a  wrong  reading  is  given.  Thus, 
the  liquid  in  the  Pitot  tube  shows  zero  when  a  fast  spiral  is  made 
although  the  speed  may  be  60  or  more 
miles  an  hour. 

The  Pitot  tube  air-speed  indicator. — The 
tube  consists  essentially  of  the  following: 
A  glass  tube  "A"  is  placed  in  the  middle 
of  a  U  tube  "B"  and  is  connected  to  it  by  a 
very  small  pipe  "C."  The  U  tube  "B"  is 
connected  to  the  pressure  tube  of  the  head. 
The  glass  tube  is  connected  to  the  static 
part.  The  tubes  are  filled  with  a  colored 
liquid  which  reads  "zero"  on  the  scale 
"  D  "'  when  the  machine  is  at  rest.  When 
the  machine  is  in  flight  the  pressure  acts 
on  the  liquid  in  the  tube  "  B  "  and  forces  it  up  the  glass  tube  so  that 
it  registers  the  speed  which  is  read  off  from  the  scale.  The  small 
tube  "C  "  is  in  order  to  keep  the  liquid  fr<;)m  moving  up  and  down  too 
quickly  so  that  a  reading  can  easily  be  taken.  The  disadvantage 
of  this  is  that  the  instrument  does  not  register  quick  changes  of  speed. 
There  are  two  branches  to  the  tube  "B,"  so  that  when  the  machine 
tilts  the  liquid  rises  in  one  branch  and  falls  in  the  other,  but  the 
reading  of  the  liquid  in  the  glass  tube  is  not  altered. 

This  instrument  is  affected  by  centrifugal  force  when  a  turn  is  made. 
The  Venturi  tube.- — The  head  of  this  tube  consists  of  a  pipe  bent  to 
face  the  direction  of  flight  and  a  second  tube  which  is  fitted  into  a 
suction  arrangement  shaped  thus: 

This  head  is  connected  to  a  suitable  gauge  so  that  the  air  speed 
can  be  read  by  means  of  a  pointer. 


AIR  SERVICE  HANDBOOK. 


103 


Fig.  37. 


The  advantage  of  this  type  of  head  is  that  the  constant  "k"  is 
five  times  as  great  as  that  of  the  Pitot  type  head.  This  means  that 
it  is  easier  to  make,  small  errors  making  only  one-fifth  the  difference. 

Types  of  gauge. — There  are  three  types  in  general  use  at  present. 
Any  one  type  can  be  used  for  nay  type  of  pressure  head  but  it  must 
be  remembered  that  the  readings  will 
have  to  be  changed. 

The  first  type  consists  of  a  metal 
box  across  which  is  stretched  a  rubber 
diaphragm,  thus: 

The  pressure  part  of  the  head  is  led 
to  the  gauge  above  the  diaphragm 
and  the  static  part  of  the  head  to  the 
part  l)elow.  Varying  pressures  make 
the  diaphragm  m(A-e  up  and  down  and  these  movements  correspond 
to  the  speed  of  the  airplane.  The  center  of  the  diaphragm  is 
connected  to  a  needle  by  means  of  a  fine  silk  cord  so  that  the 
needle  points  to  the  number  on  the  scale  corresponding  to  the  air- 
plane speed.     The  needle  is  brought  back  to  the  "zero"  position 

by    means    of    a    small,    spiral 
fo-iy,  f<.r     ^^  spring. 

This  instrument  lags  because 
a  certain  amount  of  air  has  to 
find  its  way  into  the  box  before 
the  needle  moves. 
This  gauge  is  not  suitable  for 
hot  countries  because  the  rubber  \  ery  soon  begins  to  rot  and  this 
alters  the  reading. 

The  second  type  is  much  the  same  as  the  one  above  except  that 
the  diaphragm  is  made  of  metal  and  it  is  connected  to  the  pointer  by 
a  system  of  levers. 

This  instrument  is  affected  by  changes  in  temperature  which 
make  the  diaphragm  more  or  less  elastic. 

The  third  type  of  instrument  works  on  the  same  principle  as  the 
above.     It  consists  essentially  of 


H:^-^ 


T^u.l.h« 


Fig.  38. 


1  Lt^/fj 


>>\<.l.-al.      5i,"&.)jK  fC^T 


Fig.  :;9. 


a  small  metal  box  similar  to  that 
which  is  used  in  aneroid  barom- 
eters. This  box  is  connected  to 
the  pressure  part  of  the  pressure 
head.  This  metal  box  is  fixed 
inside  the  main  case  of  the  in- 
strument, which  is  air-tight  and  connected  to  the  suction  part  of 
the  pres.-!ure  head. 

.\s  the  small  metal  box  expands  or  contracts  it  moves  the  pointer 
of  the  instniment  backward  and  forward  by  means  of  a  small  rack 
and  pinion. 


104  AIR  SERVICE  HANDBOOK. 

The  use  of  air-speed  indicators  on  airplanes. — Ks  far  as  the  pilot  is 
concerned,  it  does  not  matter  what  figure  the  air-speed  indicator  in 
the  machine  registers.  This  figure  alters  with  height  very  consid- 
erably, and  is  also  affected  by  temperature.  The  instruments  do 
not  register  small,  quick  changes  in  speed  and  ai"e  not  useful  in  bomb 
dropping  because  they  do  not  register  the  ground  speed.  Where  they 
are  exceedingly  important  is  in  night  flying,  flying  in  clouds,  and 
when  it  is  necessary  to  get  the  maximum  climb  from  the  machine. 
Every  pilot,  by  trial,  knows  the  speed  of  his  airplane  when  flying  level , 
80  that  at  night  he  can  tell  with  fair  accuracy  how  his  machine  is 
flying.  No  pilot  can  tell  by  the  feel  of  his  machine  when  he  is 
doing  the  quickest  climb,  but  by  reading  his  watch  and  aneroid 
he  knows  what  his  air-speed  indicator  ought  to  register,  so  that  after 
the  first  trial  he  can  simply  keep  his  machine  steady  at  this  speed 
in  order  to  insure  that  his  machine  is  rising  as  swiftly  as  possible. 
When  the  air  speed  indicating  the  quickest  climb  has  been  found 
this  figure  is  correct  for  every  height,  although  the  correct  reading 

of  the   instrument    alters  with    the 
'^'•'^   *■'    ^  height.     This  is  because  the  angle  of 

the  machine  has  to  be  increased  con- 
siderably at  the  higher  altitudes  in 
order  to  get  the  best  performance. 

The  compass — General  description.— 
Tire  compass  is  constructed  on  the 
principle  of  suspending  a  magnet  (or 
system  of  magnets  fixed  parallel  to  each  other  and  referred  to  as  the 
"compass  needles")  in  such  a  manner  that,  remaining  horizontal, 
they  are  free  to  take  up  the  direction  in  which  the  magnetism  of 
the  magnetic  pole  directs  them.  This  direction  is  called  the  "mag- 
netic meridian." 

A  circular  graduated  card  called  the  "compass  card"  is  fixed  to 
the  compass  needle  so  that  one  diameter  of  the  card,  the  opposite 
extremities  of  which  are  marked  north  and  south,  respectively,  is 
in  the  same  line  as  the  direction  of  the  needles.  The  point  marked 
"north  "  (in  land  and  water  compasses  distinguished  by  a  fleur-de-lis 
or  other  special  mark)  is  placed  over  that  end  of  the  needle  which 
always  points  to  the  northward.  The  extremities  of  that  diameter 
which  is  at  right  angles  to  the  north  and  south  line  are  marked  east 
and  west,  east  being  to  the  right  hand  when  the  observer  is  facing 
to  the  northward.  The  compass  card  is  thus  divided  into  four 
quarters  of  a  circle,  or  quadrants,  and  the  points  thus  obtained  are 
called  the  "cardinal  points."  These  divisions  may  again  be  sub- 
divided into  the  half  and  quarter  cardinal  points. 


AIR   SERVICE   HANDBOOK.  105 

In  the  center  of  the  conipa-a  card  a  .small  eemicircular  cap  is 
fitted  slightly  hollowed  on  it.s  underneath  side.  This  supports  the 
card  by  resting  on  a  sharp- pointed  pivot  made  of  very  hard  metal. 
Thus  the  card  is  suspended  on  an  almost  frictiouless  point  and  ia 
free  to  maintain  its  direction  when  the  airplane  is  turned. 

The  pivot  itself  is  fixed  to  the  center  of  a  bowl  called  the  "compass 
bowl."  This  bowl  is  covered  by  a  glass  plate  or  a  hole  is  cut  in  it 
to  which  the  glass  plate  is  fixed. 

All  airplane  compasses  are  of  the  liquid  type;  that  is,  the  bowl  is 
filled  with  water  and  spirits.  This  takes  the  weight  of  the  card  off 
the  pivot,  so  that  the  compass  becomes  very  sensitive.     It  also  makes 


the  needle  come  to  i"est  quickly,  so  that  it  very  soon  points  to  the 
north  after  a  turn  has  been  made. 

On  the  bowl  of  the  compass  ^vill  be  found  a  mark  called  the 
"lubber's  point,"  and  when  mounting  a  compass  in  an  airplane  this 
point  should  be  in  the  fore-and-aft  line  of  the  airplane  and  pointing 
directly  ahead.  Thus  it  follows  that  the  "lubber's  point"  moves 
with  every  turning  movement  of  the  au-plane,  and  to  ascertain  the 
direction  of  the  airplane's  head  the  observer  has  only  to  notice 
what  point  on  the  compass  card  corresponds  with  the  "lubber's 
point."  This  bearing  is  called  the  "compass  course,"  or  direction 
in  which  the  airplane  is  being  steered.  The  "compass  course  "  may 
also,  as  an  alternative,  be  described  as  the  angle  made  by  the  point 
of  the  compass  coinciding  with  the  "lubber's  point"  and  the  north 
point. 


106 


AIE   SESVICE   HANDBOOK, 


Airplane  compasses  are  usually  marked  at  the  half  cardinal  points 
and  the  card  is  graduated  in  degrees  reading  from  north  to  360 
clockwise,  i.  e.,  east  will  be  90°,  south  180°,  and  west  270°.  As 
the  cards  are  very  small,  it  is  usual  only  to  mark  every  20°,  and  the 
last  figure  "0"  is  left  out  for  ease  in  reading.  It  is  very  hard, 
indeed,  to  steer  an  airplane  within  5°  of  the  required  course,  so  that 
the  graduations  are  not  abnormally  large. 

There  is  another  way  of  designating  compass  bearings,  but  it  is 
not  used  in  the  land  services.  This  method  is  to  describe  a  bearing 
assomany  de-^ree?  ea.stor  wer't  of  north  or  south;  that  is,  southwest 
would  be  described  as  S.  45°  W.,  etc. 

Errors  to  which  compasses  are  subject. — The  compass  is  unfortu- 
nately affected  by  errors.  The  ones  chiefly  concerning  aviation 
are — 

A.  Variation. 

B.  Deviation. 

C.  Dip. 

D.  Air  bubbles  in  liquid. 

Table  of  compass  bearings. 


Bearing. 

De- 
grees. 

Bearing. 

De- 
grees. 

Bearing. 

De- 
grees. 

Bearing. 

De- 
grees. 

N 

0 
11} 

22J 

33| 

45 

56} 

67i 

78f 

E 

90 
101} 
112^ 
123J 
135 
1461^ 
1574 
1682 

S    

180 

191} 

202.V 

213| 

225 

236} 

247i 

2581 

W 

W.by  N.. 
W.-N  W. . . 
NW.byW. 

NW 

NW.bvN. 
N.-NW... 
N.by  W.. 

270 

N. by  E . . . 
N.-NE.... 
NE.by  N.. 

NE 

NE.byE.. 

E.-NE 

E.byN... 

E.bvS... 
E.-SE  .... 
SE.by  E.. 
SE 

SE.bvS.. 

S.-SE 

S.byE... 

S.  by  W. . . 
S.-SW.... 
SW.by  S.. 

SW 

SW.by  W. 
W.-SVV... 
W.by  S... 

281} 

292i 

303i 

315 

326} 

337* 

348i 

The  above  are  called  th?  "points"  of  the  compass.  Each  "point" 
is  equivalent  to  an  angle  of  11^  degrees. 

A.  Variation<i. — A  suspended  magnet  or  compass  needle  does  not 
point  to  the  true  or  geographical  north  but  to  a  point  known  as  the 
magnetic  North  Pole.  Th<^  difference  between  this  direction  and 
the  direction  of  ths  true  north  is  called  the  "variation  of  the  com- 
pass" or  shortly  "variation.'' 

The  variation  changes  according  to  ones  position  on  the  earth's 
surface  and  also  annually;  the  latter  is  very  gradual  but  the  former 
must  never  be  ignored.  V^ariation  is  measured  in  degrees  to  the 
east  and  west  of  true  north  at  Greenwich;  at  the  present  time  the 
compass  needle  points  about  15°  to  tlie  west  of  true  north  at  Green- 
wich and  as  one  goes  west  it  gradually  increass  to  30°  and  then  de- 
creases until  about  the  middle  of  the  State  of  Ohio  there  is  no  varia- 
tion and  still  further  west  the  variation  changes  to  easterly. 


AIR  SERVICE   HANDBOOK.  107 

The  variation  on  the  east  coast  of  America  near  Washington  ia 
about  8°  west  and  the  variation  in  California  about  14°  east. 

Near  Gi'een\\ich  the  variation  is  decreasing  by  about  W  annually. 

The  actual  variation  can  be  obtained  from  the  Ordnance  Survey 
Maps.  These  maps  are  made  out  on  the  true  north  and  south  prin- 
ciple so  that  a  course  taken  from  them  would  be  a  "'true  course" 
and  the  variation  must  be  applied  before  a  'magnetic  course"  \& 
obtained. 

B.  Deviation. — -This  is  due  to  the  local  attraction  of  steel  and  iron 
fittings  in  the  immediate  vicinity  of  the  compass;  it  varies  both  in 
magnitude  and  direction  for  different  positions  of  the  airplane.  It 
therefore  ^\^.ll  be  readily  understood  that  it  is  necessary  to  place  a 
compass  in  an  airplane  in  such  a  position  as  to  be  as  far  as  possible 
free  from  these  influences.  This,  however,  is  a  difficult  matter;  but 
so  long  as  the  compass  is  not  affected  to  a  greater  extent  than  about 
5°  the  error  can  remain  uncorrected,  as  for  practical  work  it  will  be 
found  difficult  to  steer  an  airplane  accurately  enough  for  this  amount 
of  error  to  seriously  matter.  If,  on  the  other  hand,  the  error  is  con- 
siderably in  excess  of  5°  it  should  be  the  work  of  an  expert  to  cor- 
rect it  by  means  of  magnets  placed  near  the  compass  in  such  a  manner 
as  to  counteract  the  local  influence  on  the  needle.  !Most  compasses 
carry  an  arrangement  to  hold  the  small  "compensating  magnets'" 
both  in  the  fore  and  aft  line  of  the  machine  and  also  transversely. 
Should  a  pilot  be  of  the  opinion  that  his  compass  i-^  considerably 
"out"  on  a  certain  course  his  best  method  is  to  point  the  airplane  at 
a  distant  object  situated  on  or  near  that  course,  start  up  th(^  engine 
and  note  the  reading  of  the  card,  at  the  same  time  taking  a  bearing 
of  the  object  by  means  of  another  compass  (known  to  be  free  from 
local  error)  placed  in  line  with  the  airplane  and  object,  but  at  some 
distiuce  away.  By  comparing  the  two  bearings  obtained  the  pilot 
can  at  once  ascertain  the  error  and  if  imable  to  correct  it  by  magnets 
he  must  remember  to  apply  it  to  his  "magnetic  course." 

This  method  can  be  employed  for  all  directions  of  the  airplane 
and  a  table  made  out  showing  the  deviation  for  every  10°  or  20°; 
but  it  is  more  satisfactory  if  the  pilot  can  have  his  compass  properly 
corrected  when  it  is  first  fitted  on  the  airplane  so  that  the  only  error 
he  has  to  apply  is  variation. 

Every  machine  which  has  passed  the  inspection  dei)artment  car- 
ries a  small  plate  on  which  are  marked  the  actual  compass  readings 
at  each  of  the  half  cardinal  points. 

C.  Dip. — The  earth's  lines  of  magnetic  force  are  not  horizontal 
except  near  the  Equator  and  they  vary  at  every  latitude  imtil  at 
the  magnetic  North  Pole  a  compass  needle  free  to  swing  up  and 
dowTi  would  point  directly  down  toward  the  earth.     On  account  of 


108  AIR  SERVICE  HANDBOOK. 

this  tendency  of  the  needle  to  dip  it  is  necessary  to  fix  a  small  weight 
on  the  south  end  of  the  needle  or  compass  card  (in  the  Northern 
Hemisphere)  so  as  to  make  it  hang  horizontal.  If  the  needle  of  the 
compass  is  strongly  magnetic  this  small  weight  has  to  be  compara- 
tively heavy. 

D.  Air  bubbles  in  liquid. — An  air  bubble  is  a  large  factor  in  pro- 
ducing inaccuracy  in  a  compass.  If  the  bubble  is  sufficiently  big 
the  vibration  will  cause  the  liquid  to  froth  and  the  card  will  become 
illegible.  At  the  same  time  the  friction  caused  by  the  bubble 
moving  in  the  bowl  will  tend  to  deviate  the  card  from  the  magnetic 
course.  In  some  cases  it  will  even  cause  the  card  to  revolve  com- 
pletely around. 

If  it  is  desired  to  get  rid  of  this  bubble,  the  compass  bowl  should 
be  turned  on  its  side  until  the  filling  hole  is  at  the  top.  Remove 
the  plug  and  fill  the  bowl  carefully  with  distilled  water,  or,  better 
still,  with  the  special  compass  liquid.  During  this  operation  care 
must  be  taken  to  insure  that  the  passage  of  ah"  out  of  the  bowl  is 
not  hindered  by  the  presence  of  drops  of  liquid  in  the  plug  hole. 
When  the  bowl  is  completely  full  to  the  top  of  the  plug  hole  replace 
the  plug. 

AIRPLANE    COMPASSES   USUALLY   MET   WITH. 

A.  The  naval  and  military  airplane  compass. — In  this  compass  the 
bowl  rests  in  its  outer  casing  on  a  horsehair  pad  and  is  kept  in  place 
by  three  trunnions  with  rubber  rings  which  fit  into  brackets  on  the 
inside  of  the  case.  The  horsehair  is  in  order  to  j^revent  the  bowl 
vibrating  too  much. 

The  bowl  is  filled  with  a  mixture  of  three  parts  distilled  water  to 
one  part  of  alcohol.  The  bowl  is  filled  through  a  filling  plug  (fitted 
with  a  brass  screw)  situated  on  one  side. 

The  suspension  of  the  compass  card  is  arranged  as  follows: 

In  the  center  of  the  card  there  is  a  cii'cular  cap  inside  which  is 
fixed  an  amethyst.  This  amethyst  rests  on  a  pivot  fixed  to  the 
bottom  of  the  bowl  and  having  an  iridium  pointed  tip.  Attached 
to  the  underside  of  the  card  and  3^  inch  from  it  there  are  two  mag- 
nets 2|  inches  long  by  3%  inch  diameter.  The  centers  of  these  mag- 
nets are  If  inches  apart. 

The  compass  card  is  graduated  from  0°  at  north  every  10°  right 
handed  through  east  90°,  south  180°,  and  west  270°.  The  final  0 
of  each  number  is  omitted,  so  that  280°  reads  28°,  etc.,  on  this  card. 
The  card  is  also  marked  in  cardinal  and  half-cardinal  points;  the 
figures  and  letters  in  the  northern  quadrants  are  red  and  in  the 
southern  blue.  On  the  outer  rim  of  the  card  is  marked  a  set  of 
degrees  inverted,  which  are  reflected  correctly  in  a  prism  fitted 
immediately  over  the  "Lubbers  point."     The  bracket  for  this  prism 


AIR   SERVICE  HANDBOOK. 


109 


is  fixed  to  the  Ijowl.  hut  the  prism  itself  can  be  moved  to  suit  the 
individual  user  and  his  vision  relative  to  the  compass. 

The  "Lubbers  point"  is  of  brass  wire,  in  the  shape  of  a  right  angle 
triangle,  only  two  sides  of  which  are  seen — one  when  viewed  directly 
and  the  other  when  the  prism  is  u.sed. 

On  the  outer  rim  of  the  bowl  is  a  movable  circumference  having 
a  vdre  diameter  across  the  top  of  the  bowl.  This  wire  is  called  the 
"course  pointer,"  and  can  also  be  used  to  assist  the  reflection  of 
the  "Lubbers  point,"  when  the  former  is  laid  coincidental  ^^'ith  it. 

Let  into  the  bottom  of  the  bowl,  immediately  under  the  "Lubbers 
point"  and  prism,  is  a  one-half  inch  diameter  circle  of  opaque  glass. 
Inside  a  holder  which  can  be  placed  on  the  various  quadrants  of 


A/isgnets. 


Co  ^ pass 


A^ar'/fir? 


Fig.  42. 


the  case  is  a  dry  cell — 1.3  volts,  0.3  ampere — which  lights  a  0.5- 
candlepower  clear-})ulb  lamp  fixed  inside  the  case  and  under  the 
bowl.  Tlie  light  from  this  lamp  shines  through  the  opaque  glass 
and  up  into  the  prism.  The  'Lubbers  point"  and  also  the  degrees 
on  the  card  reflected  to  the  prism  are  thus  illuminated  so  that  the 
compass  can  be  used  at  night. 

At  the  bottom  of  the  bowl,  covered  by  a  light  plate  held  in  place 
l)y  four  nuts,  is  an  expansion  chamber.  This  allows  for  the  liquid 
becoming  heated  by  the  lamp,  or  by  differences  in  atmospheric 
temperature.  This  chamber  can  also  be  used  for  getting  rid  of  any 
air  bubble  which  may  be  in  the  liquid. 

B.   The  K.  A.  F.,  Mark  II,  compass.— 

This  compass  consists  of  a  spherical  bowl  which  rests  in  a  case  on 
a  pad  of  horsehair  and  is  connected  to  it  by  bolts  and  rubber  washers. 


110 


AIR   SERVICE   HANDBOOK. 


A  circular  window  is  placed  in  the  side  of  the  bowl  toward  the  top, 
so  that  the  card  can  be  seen.  The  bowl  carries  an  expansion  chamr 
ber  at  the  bottom  and  a  holder  for  a  small  electric  light  at  the  top. 
The  outer  case  has  a  fitting  to  take  the  standard  compensating 
magnets  attached  to  its  underneath  side. 

The  compass  card  consists  of  a  circular  disk  which  is  mounted 
on  a  pivot  attached  to  the  bottom  of  the  bowl.  This  disk  contains 
two  magnets,  and  it  is  prevented  from  lifting  off  the  pivot  by  a  wire 
attached  to  the  side  of  the  bowl.  Fixed  into  the  disk  are  two  wires 
at  right  angles  which  carry  on  their  ends  a  ring  on  which  the  mark- 
ings are  borne.  This  ring  is  marked  on  the  inside  with  the  cardinal 
and  half-cardinal  points.  It  is  also  graduated  every  20°,  the  last 
0  of  each  number  l)eing  omitted  for  the  sake  of  clearness. 


This  compass  has  short  and  weak  magnets  so  that  the  card  has  a 
long  period  of  oscillation.  The  card  of  this  compass  is  little  affected 
when  the  airplane  turns. 

C.  The  Creigh  Osborne  compass. — This  compass  consists  of  a  spheri- 
cal bowl  which  has  a  window  similar  to  that  mentioned  above.  The 
bowl  is  supported  in  three  places  by  lugs  which  rest  on  brackets 
projecting  from  a  frame  and  to  which  they  are  attached  by  bolts 
and  felt  washers.  On  top  of  the  bowl  is  a  fitting  to  take  the  com- 
pensating magnets.  This  compass  has  no  expansion  chamber,  and  a 
bubble  of  air  is  purposely  left  inside  the  bowl  to  allow  for  expansion 
of  the  licjuid.     The  card  is  supported  in  the  usual  manner  to  a  pivot 


AIR   SERVICE   HANDBOOK.  Ill 

fixed  to  the  bottom  of  the  bowl  ami  is  prcvcnttMl  inun  lallintr  <'if 
by  a  wire  attached  to  the  top  of  the  Ixtwl.  Two  mai^nets  arc*  (ixed 
to  the  underneath  side  of  the  card,  and  the  outer  edpje  of  the  card  is 
turned  up  at  an  angle.  This  turne<l-up  part  carries  the  markings. 
On  the  inside  it  is  graduated  every  20°  in  the  name  manner  as  the 
compass  mentioned  above,  and  it  als)  is  marked  with  the  <-ardinal 
and  half-cardinal  points.  The  outside  of  this  turne<l-up  i^art  of 
the  card  is  marked  also  with  the  cardinal  and  half-cardinal  points. 
At  the  bottom  cf  the  circular  window  arc  two  wires  which  f  ro.ss  at 
right  angles;  the  vertical  one  is  the  "Lubliers  jxiint  "  and  the  hori- 
zontal one  is  to  aid  in  keeping  the  air])laiie  horizontal.  This  i-onipa-^s 
has  long  and  comparatively  strong  magnetos  and  has  therefor*'  a 
short  period  of  oscillation. 

Action  of  compass  uhen  flying  through  clouds  or  at  night,  ll  used 
to  be  said  that  clouds  were  "magnetic"  because  pilots  found  that 
the  c6mpass  card  would  swing  apparently  of  its  own  accord  as  soon 
as  the  machine  entered  the  clouds.  This  was  observed  when  the 
compass  alone  was  used  to  steer  by  and  it  was  not  always  observed. 
The  clouds  which  are  usually  met  on  a  flying  day  are  not  "magnetic." 
The  electricity  in  the  black  thunder  clouds  may  affect  the  compass 
needle  but  one  does  not  usually  fly  in  thunder  storms.  The  explana- 
tion of  the  8%vinging  of  the  compass  card  is  as  follows: 

As  has  been  mentioned  above,  the  north  end  of  the  magnetic 
needle  tends  to  dip  in  the  Northern  Hemisphere  and  the  amount  of 
the  inclination  depends  upon  the  latitude.  This  tendency  is 
counteracted  by  placing  a  small  weight  on  the  south  end  of  the 
needle  in  order  to  make  it  hang  level.     (Fig.  44,  a.) 

Suppose  a  machine  was  flying  north  and  is  turned,  either  because 
the  pilot  wants  to  turn  or  because  he  does  not  know  that  he  is  turning. 
As  soon  as  the  machine  leaves  its  course  centrifugal  force  acts  on  the 
Compass  needle  and  the  card  being  hung  from  the  point  of  the  pivot 
will  wing  outward  and  take  up  the  angle  corresj)onding  to  the  turn. 
As  the  card  is  supported  at  one  point  only  it  will  tend  to  take  up 
its  proper  angle  no  matter  if  the  machine  is  making  a  turn  with  the 
proper  bank  or  if  it  is  skidding  round. 

If  the  turn  is  made  to  the  left  (west)  one  would  imagine  that  the 
card  would  swing  out  and  that  the  needle  would  still  point  to 
the  north  and  that  the  "Lubbers  point"  would  move  to  the  left, 
thus  showing  a  turn  to  the  left.  (Fig.  44,  b.)  In  most  compas;se8  it 
does  not  do  this  and  this  is  what  actually  happeiLs.  The  card  swings 
outward,  but  the  south  end  being  heavier  than  the  north  will  swing 
with  greater  vigor,  so  that  the  north  end  of  the  card  is  swung  inward 
and  the  "Lubbers  point,"  instead  of  moving  to  the  left,  moves  rela- 
tively to  the  right  and  registers  apparently  a  right-hand  turn. 


112 


AIR   SERVICE   HANDBOOK. 


t 


If  one  were  steering  by  compass  alone  the  pilot  would  put  on  the 
"left  rudder"  and  the  error  would  be  increased.  Very  soon  the  pilot 
would  have  lost  all  sense  of  direction  and  the  card  might  swing 
completely  round,  as  it  often  does.  If  the  pilot  has  any  point  to 
steer  on  the  swinging  is  put  down  to  the  ordinary  lag  of  the  com- 
pass and  the  pilot  holds  the  ma- 

T  ''^''         T  chine  steady   until  the  card  has 

1  i  I  _  ^ '  settled. 

^,      ^  nT,  It  might  be  thought  that  this  er- 

ror could  be  adjusted  by  altering 
the  weights  of  the  compass  card, 
but  this  can  not  be  done.  Consider 
the  forces  acting  on  the  needle 
from  another  point  of  view.  As 
soon  as  the  card  tilts  the  resultant 
forces  affecting  the  needle  (all  ex- 
cept magnetic  attraction)  act  in  a 
plane  at  right  angles  to  the  plane 
of  the  compass  card  (which  is  not 
horizontal).  The  magnetic  force 
always  tends  to  pull  the  needle  so 
as  to  make  it  lie  parallel  to  the 
earth's  lines  of  magnetic  force,  and 
this  means  that  there  is  always  an 
unbalanced  force  tending  to  pull 
the  north  end  of  the  compass  nee- 
dle vertically  toward  the  earth; 
i.  e.,  toward  the  inside  of  the  turn 
and  not  outward  as  one  might  ex- 
pect. The  only  way  of  correct- 
ing for  this  error  is  to  make  the 
magnets  weak  so  that  there  is  very 
little  magnetic  force  and  therefore 
very  small  extra  weight  on  the 
south  end  of  the  needle  acting 
unbalanced  in  a  turn. 

When  flying  south  the  heavy  end 
of  the  needle  will  still  tend  to  fly 
outward  on  a  turn,  but  in  this  case  the  error  will  be  in  the  proper 
way;  that  is,  if  the  machine  turns  toward  the  west  the  error  will  also 
be  toward  the  west  and  the  pilot  can  easily  see  which  way  to  put 
the  rudder. 

Pilots  should  examine  their  compasses  and  find  out  if  they  are  of 
the  type  which  will  tend  to  turn  the  wrong  way  or  the  right  way 


Fig.  44. 


AIR  SERVICE  HANDBOOK. 


118 


Fig.  45. 


when  the  machine  is  turning  north.  Those  compa.sses  with  a  short 
period  are  likely  to  be  wrong;  those  with  the  long  period  will  probably 
be  right. 

If  the  compass  is  of  the  wrong  type  and  it  is  found  necessary  to  fly 
through  clouds,  the  machine  should  be  pointed  south  until  it  ia 
clear  of  the  clouds.  As  the  com- 
pass card  takes  a  certain  time  to 
settle  down  after  a  turn  any  turning 
necessarj'^  should  be  completed  at 
least  one  minute  before  the  machine 
reaches  the  cloud. 

AIK-PHE.SSUHE  GAUGE. 

Air-pressure  gauges  usually  con- 
sist of  a  flat  tube  of  thin  metal 
bent  round  in  the  form  of  a  circle, 
one  end  is  connected  by  a  pipe  to 
the  tank,  the  other  end  is  closed  and  attached  by  means  of  a  small 
link  to  the  end  of  a  pointer.  The  principle  of  the  gauge  is  this :  When 
a  bent  tube  is  acted  on  by  pressure  from  the  inside  it  tends  to 
straighten.     The  scale  of  the  instrument  can  easily  be  graduated 

with  reference  to  a  standard 
pressure  gauge. 

Revolution  indicators. — The 
most  usual  form  of  revolution 
indicator  for  the  engine  con- 
sists essentially  of  two  weights 
which  when  rotated  fly  out- 
wards, the  amount  corre- 
sponding to  the  speed  of  the 
engine. 

The  weights  of  the  indi- 
cator, two  or  three  in  number, 
are  attached  to  the  middle  of 
springs,  one  end  of  the  springs 
are  attached  to  the  end  of  a 
vertical  spindle,  the  other 
ends  are  attached  to  a  col- 
lar which  is  free  to  move  up  and  down  the  spindle.  This  spindle  is 
rotated  by  means  of  a  flexible  drive  by  some  suitable  part  on  the 
engine  such  as  the  pump  spindle.  If  necessary  a  gear  box  is  attached 
between  the  drive  and  the  indicator  so  that  the  indicator  will  show 
suitable  readings.    The  movable  collar  on  the  spindle  is  grooved 


46643—18- 


-8 


114  AIR  SERVICE  HANDBOOK. 

and  these  grooves  fit  into  the  teeth  of  an  arc  carried  on  the  axis  of 
the  pointer  so  that  when  the  weights  swing  outward  the  collar  is 
drawn  up  the  spindle  and  this  rotates  the  arc  and  therefore  the 
pointer.  The  pointer  thus  points  to  the  number  of  revolutions 
corresponding  to  the  amount  the  weights  have  swung  outward.  As 
the  speed  of  the  engine  lessens  the  weights  are  drawn  in  toward  the 
spindle  by  means  of  the  springs  to  which  they  are  attached.  Small 
lubricating  cups  are  carried  at  each  end  of  the  spindle  and  these 
should  be  oiled  from  time  to  time.  The  flexible  drive  should  not 
be  led  round  sharp  corners  and  it  should  not  be  fixed  in  small  curves, 
otherwise  it  will  be  broken  and  this  especially  applies  to  the  end 
where  it  is  attached  to  the  indicator  or  the  engine. 

Levels. — All  machines  carry  a  level  to  show  if  the  machine  is 
flying  one  wing  high  and  some  machines  also  carry  a  fore-and-aft  level. 
These  are  usually  of  the  ordinary  liquid  type;  the  tube  is  slightly 
curved  so  that  the  bubble  shows  on  the  top  of  the  curve.  These 
levels  are  only  correct  when  the  machine  is  flying  on  a  steady  course. 
If  the  machine  makes  a  turn  with  the  proper  bank  the  bubble  will 
stay  in  the  center  and  register  no  inclination  of  the  wings.  If  the 
turn  is  too  flat  the  liquid  will  be  pulled  outward  by  centrifugal  force 
and  the  bubble  will  show  toward  the  inside  of  the  curve  and  this 
gives  apparently  a  wrong  reading.  If  the  machine  is  banked  up 
too  much  for  the  turn  the  bubble  will  show  on  the  outside  of  the 
curve.  The  aiiplane  should  be  flown  therefore  so  that  the  bubble 
of  the  transverse  level  is  always  in  the  center.  In  the  same  manner 
the  fore-and-aft  level  will  only  register  correctly  when  the  machine 
is  flying  in  a  straight  line  and  will  not  be  correct  when  the  machine 
is  changing  to  a  climb  or  glide. 

INSTRUMENTS    FOR    SHOWING    MAXIMUM    C'UMB. 

There  is  a  simple  instrument  for  showing  when  the  machine  is 
doing  its  best  climb  and  it  consists  essentially  of  the  following:  A 
vacuum  flask,  something  similar  to  that  of  the  thermos  bottle,  has 
two  tubes  leading  out  of  it.  The  fu-st  tube  is  almost  closed,  only  a 
small  hole  being  left.  The  other  tube  is  connected  to  a  U  tube 
containing  a  liquid.  ^Vhen  the  machine  climbs  the  pressure  of 
the  air  inside  the  bottle  becomes  greater  than  the  air  outside.  The 
air  inside  therefore  expands,  pushing  the  liquid  up  one  branch  of 
the  U  tube,  at  the  same  time  the  air  excapes  through  the  small  hole 
in  the  first  tube.  When  the  machine  is  doing  its  best  climb  the 
difference  of  pressure  between  the  air  inside  the  bottle  and  the  air 
outside  becomes  a  maximum  and  the  liquid  is  pushed  as  high  as  it 


AIR  SERVICE  HANDBOOK. 


115 


can  be  up  the  branch  of  the  U  tube.  The  machine  shouhl  there- 
fore be  climbed  with  the  liquid  a.s  high  as  possible  in  the  U  tube. 
^Vhen  this  is  being  done  the  reading  on  the  air-speed  indicator  should 
be  taken  and  noted  for  future  u.se.     The  bottle  is  of  the  vacuum 


5»n*U  ?r«<!«. 


-   Ptalt 


type  so  as  to  prevent  the  difference  in  temperature  of  the  air  out- 
side affecting  the  air  inside  the  l)ottle. 

INSTRUMENT    FOR    TELLINTr    WHEN'    THE    M.\CHINE    IS    FLVING    LEVEL. 

There  is  a  simple  instrument  for  telling  when  the  machine  is 
flving  level,  which  depends  on  the  following  principle:  A  vacuum 


n 


v. 


Fig.  48. 


flask  .similar  to  that  used  in  the  instrument  above  has  also  two  tubes 
leading  out  of  it,  one  of  which  can  be  opened  or  closed  at  will  and 
the  other  leading  to  a  horizontal  tube  which  is  straight  or  may  be 
slightly  bent.     In  the  middle  of  this  horizontal  tube  is  a  drop  of 


116  AIR   SERVICE   HANDBOOX. 

colored  liquid.  This  drop  clings  to  the  sides  of  the  tube  and  if 
there  is  any  difference  of  pressure  between  the  air  outside  the  bottle 
and  that  inside  the  bottle  it  will  move  back  and  forth  in  the  tube. 
If  the  first  tube  is  closed  and  if  the  machine  is  flying  level  the  little 
drop  of  liquid  will  be  in  the  center  of  the  tube  and  will  not  move- 
Directly  the  machine  starts  to  go  up  or  down  there  will  be  an  immedi- 
ate change  of  pressure  between  the  air  outside  the  bottle  and  that 
inside  so  that  the  drop  of  liquid  will  move  one  way  or  the  other. 
There  is  a  small  baffle  at  each  end  of  the  tube  to  prevent  the  liquid 
being  sucked  into  the  bottle  or  being  blown  out  altogether.  When 
the  first  tube  is  opened,  if  the  horizontal  tube  is  slightly  bent,  the 
liquid  will  return  to  the  center  of  its  run.  This  instrument  can  be 
made  with  a  diaphram  and  with  a  suitable  gearing  between  the 
diaphram  and  a  pointer  to  magnify  the  movements  of  the  diaphram. 

XIII.  NAVIGATION  OF  THE  AIR. 

General  remarks. — Accurate  navigation  is  obtained  by  intelligent 
use  of  a  compass  combined  with  a  good  knowledge  of  topography  to 
assist  in  rapidly  locating  the  position. 

The  greatest  difficulty  is  experienced  by  pilots  in  finding  their 
way  across  country  at  the  first  attempt  even  if  the  locality  is  well 
known  from  below.  The  country  presents  a  different  aspect  when 
viewed  from  above,  and  only  by  constant  practice  can  a  pilot  become 
what  is  known  as  a  "good  cross-country  flyer." 

The  secret  of  success  in  navigating  an  airplane  is  careful  attention 
to  details.  The  pilot's  task  is  made  considerable  easier  if  he  has  a 
trained  observer  as  passenger,  with  suitable  means  of  communicat- 
ing with  the  latter. 

Maps. — Pilots  must  be  well  acquainted  with  map  reading;  not 
only  must  the  pilot  be  able  to  check  the  turnings  and  the  different 
objects  as  is  done  when  using  a  map  on  the  ground,  but  he  should 
have  the  whole  of  the  country  in  his  mind.  It  should  not  be  nec- 
essary to  turn  the  map  upside  down  or  sideways  so  as  to  get  one's 
course  always  from  the  bottom  to  the  top  of  the  map.  A  little  train- 
ing will  enable  the  pilot  to  read  the  map  at  once  if  he  places  it  in  his 
case  with  the  north  toward  the  top  so  that  the  names  of  towns,  etc., 
can  easily  be  read.  Maps  should  be  neatly  folded  and  placed  in 
the  map  case.  They  may  be  marked  with  a  soft  lead  pencil  so  that 
the  marking  can  easily  be  rubbed  out,  but  it  is  a  mistake  to  use  ink 
on  any  map  or  to  pin  it  to  a  board,  for  this  destroys  the  map  and 
allows  it  to  become  dirty  and  torn  and  even  sometimes  to  be  blown 
away  altogether. 

For  ordinary  flights  the  most  useful  scale  is  about  3  miles  to  the 
inch  (R.  F.  1/200,000).     A  map  constructed  to  this  scale  can  be 


AIR  SERVICE  HANDBOOK.  117 

folded  so  as  to  take  iu  the  whole  area  of  the  ordinary  recounaLssaiuc. 
For  long  flights  the  map  will  have  to  be  cut  into  strips  and  mounted 
on  rollers.  When  this  is  done  it  is  convenient  to  cut  the  map  so 
that  one's  course  shows  along  the  middle  of  the  map.  It  requires 
a  little  practice  to  read  the  maps  when  this  is  done. 

For  artillery  work  and  special  duties  it  is  necessary  to  use  a  map 
of  a  much  larger  scale.  A  convenient  size  is  about  3  inches  to  1  mile 
(R.  F.  1/20,000).  These  maps  show  the  country  in  great  detail. 
They  are  usually  divided  up  into  squares  which  are  immbered  on 
a  certain  system.  These  maps  can  conveniently  ho  cut  into  squares 
and  mounted  on  thin  boards  and  then  varnished.  A  suitable  case 
should  be  provided  in  the  machine  to  take  these  maps  so  that  they 
will  not  become  damaged  or  blow  away  when  they  are  not  in  use. 

It  is  often  necessary  especially  in  a  war  to  use  foreign  maps,  the 
scale  of  which  is  usually  given  as  a  representative  fraction.  Pilots 
when  supplied  with  these  maps  should  immediately  construct  the 
corresponding  scale  with  which  they  are  familiar.  This  will  facili- 
tate rapid  calculation  of  distances  in  units  with  which  pilots  are 
accustomed  to  work. 

The  pilot  having  been  directed  to  proceed  to  a  number  o  points 
or  some  certain  point  must  study  his  map  very  closely  before  he 
gets  into  the  machine  in  order  to  ascertain  what  guides  he  can  best 
use  to  assist  liis  navigation.  If  there  is  no  side  wind  and  his  com- 
pass is  correct,  a  straight  course  from  point  to  point  is  the  quickest. 
The  points  on  the  map  should  be  joined  by  a  line  and  the  "true 
coxirse"'  measured.  To  this  the  variation  of  the  compass  in  that 
locality  must  be  applied,  and  he  then  has  the  compass  or  magnetic 
course  to  be  steered.  If  the  variation  of  the  compass  is  west  the 
variation  must  be  added  to  the  true  bearing  in  order  to  get  the  proper 
magnetic  bearing.  If  the  variation  is  east  the  variation  must  be 
subtracted  to  get  the  compass  bearing.  This  compass  bearing  should 
be  written  down  and  kept  in  some  conspicuous  position  in  front  of 
the  pilot.  The  distances  from  the  starting  point  can  also  with  ad- 
rantage  be  marked  either  at  10-raile  intervals  or  from  some  well- 
defined  object  passed  en  route  to  the  next.  Sometimes  it  is  advan- 
tageous to  mark  these  intervals  in  time,  say  every  10  minutes,  so 
that  the  pilot  ma>-  know  at  what  o'clock  he  should  be  over  certain 
places.  It  should  be  noted  whether  any  very  high  ground  is  to  be 
passed  over  necessitating  a  greater  height  being  maintained  at  that 
point. 

Selection  of  objects  us  guides. — The  following  remarks  are  the  result 
of  practical  experience: 

Towns. — Towns  are  obviously  of  the  greatest  assistance.  In  case 
of  doubt  they  are  usually  most  easily  identified  by  the  railways. 


118  AIR  SERVICE  HANDBOOK. 

No  airplane  should  pass  directly  over  a  town  as  not  only  is  such  a 
practice  contrary  to  law,  but  also  unless  flying  at  over  2,000  feet  the 
effects  of  any  large  works  with  blast  furnaces  will  be  felt.  On  hazy 
days  it  should  be  remembered  that  smoke  hangs  over  villages  and 
sometimes  gives  them  the  appearance  of  a  large  town  from  some 
little  distance  away. 

Railways. — Railways  are  of  very  great  assistance  and  can  be  used 
to  a  large  extent  as  a  guide  from  point  to  point. 

The  conventional  sign  for  a  railway  is  a  plain  black  line  on  the 
map,  and  no  distinction  is  made  between  a  line  with  perhaps  four 
paii's  of  rails  and  one  pair  of  rails.  Thus  it  is  quite  easy  to  make  a 
mistake  if  a  single  line  branches  off  from  the  main  line  in  perhaps  a 
not  too  conspicuous  place.  Branch  lines  to  quarries  are  often  not 
marked  on  the  map  even  though  they  may  run  a  mile  or  more  away 
from  the  main  line.  Tunnels,  liridges,  and  cuttings  are  marked  on 
maps  and  these  will  often  be  of  assistance  in  picking  up  the  correct 
line 

Sometimes  grass  is  allowed  to  grow  over  the  track  esi^ecially  if  it  is 
a  light  railway  and  this  makes  it  practically  invisil^le  from  a  height 
of  3,000  feet.  In  snow  a  tarred  road  which  has  had  a  little  traffic 
over  it  looks  very  like  what  a  railway  does  in  the  ordinaiy  times  and 
it  is  very  easy  to  make  a  mistake. 

In  spite  of  the  above  few  details  which  are  liable  to  cause  an  error, 
a  pilot  may  find  it  worth  his  while  to  keep  to  the  railways  and  go  a 
little  farther  round  and  this  applies  especially  in  misty  or  windy 
weather,  when  it  is  hard  to  keep  to  a  compass  course.  In  this  case 
the  general  direction  of  the  railway  should  be  noted  so  that  the  pilot 
■will  not  find  himself  followdng  the  wrong  line. 

Roads. — As  a  general  rule  roads  are  not  a  particularly  good  guide- 
Many  roads  twist  about  considerably.  Main  roads  are  often  less 
noticeable  from  a  height  than  minor  roads.  The  telegraph  wires 
and  poles  (a  sure  sign  of  an  important  road)  are  also  very  hard  to  see. 
In  the  neighborhood  of  the  fighting  line  the  places  where  troops 
have  marched  during  the  night,  even  if  they  have  gone  across  coun- 
try, look  very  like  a  permanent  road,  especially  of  the  soil  is  chalky. 

There  are  exceptions  to  the  general  rule.  Roman  roads  being 
usually  a1)solutely  straight  can  generally  he  picked  out  easily  and 
also  roads  over  a  moor  or  plain  where  there  are  few  others  in  the 
vicinity  with  which  to  confuse  them.  The  Napoleonic  roads  in 
Europe,  which  are  planted  on  l>oth  sides  with  poplars  and  which 
are  straight  for  miles,  make  very  good  landmarks. 

Water. — Water  can  be  seen  from  a  great  distance  and  is  the  best 
guide.  After  much  rain  a  pilot  must  take  into  consideration  the 
possibilities  of  a  fiooded  .stream  causing  the  siuTounding  meadows, 


AIR  SERVICE  HANDBOOK.  119 

etc.,  to  be  inundatod  lo  a  (l('))th  of  jjcrhaps  only  a  low  iiifhcs,  hut 
nevertheless  havinu:  an  appearance  of  a  t,'0()fl-sizc(l  lake  or  l)road 
river  which  can  not  be  located  on  the  map. 

Rivers  are  very  winding:  and  are  often  almost  concealed  by  high 
trees  on  either  bank.  A  pilot  will  usually  waste  time  if  he  elects  to 
follow  a  river  as  a  means  of  getting  from  jioiiil  to  ])oint.  On  most 
maps  the  smallest  rivers  are  marked  very  disiiiicily  whicli  will  at 
first  encourage  a  pilot  to  follow  them. 

Large  canals  are  easy  to  see  and  often  go  very  straight.  In  dry 
weather  the  course  of  a  river  can  be  noticed  at  once  by  the  difference 
in  color  l)etween  the  trees  near  the  river  and  those  farther  away. 

11  oor/.s.- Woods  can  be  seen  from  a  distance  and  can  often  be 
identified  very  easily  by  their  shape  or  the  .shape  of  cuttings,  but  it 
.-^hould  be  liorne  in  mind  that  it  is  very  easy  to  alter  the  shape  of  the 
woods  or  even  to  cut  it  down  altogether  so  that  the  location  can  not 
be  seen  when  flying  at  a  little  height. 

High  grouml.—  Yvom  a  height  of  2.000  feet  and  over  country 
presents  quite  a  flat  appearance  and  contour  can  not  be  recognized. 
In  early  morning  or  late  evening  hills  may  cast  a  shadow  and  stand 
out  from  the  surrounding  country.  A  pilot  should  not  fail  to  note 
any  high  ground  with  steep  contours  which  will  make  landing 
difhcult  and  he  should  fly  high  at  these  points.  The  general  lay  of 
the  country  should  be  borne  in  mind.  In  many  places  the  edges  of 
the  plain  or  downs  are  very  distinct  and  may  form  a  convenient  mark. 
On  a  long  cross-country  flight  the  color  of  the  country  will  change  on 
account  of  the  differences  in  trees  or  grass  and  this  may  aid  the  pilot 
in  case  of  doubl . 

Forced  laruinigs.  Landing  ground  is  hard  to  recognize  as  l^eing 
good  from  a  greater  height  than  LOOO  feet.  The  nicest  height  at 
which  to  fly  is  about  8,000  feet.  At  this  height  the  ground  can  be 
clearly  seen,  the  machine  is  usually  above  the  ■'])umi)y"  air,  and 
in  case  of  engine  failure  can  glide  for  some  little^  distance  Ix'fore  the 
spot  for  landing  on  need  be  finally  selected. 

The  best  time  of  year  for  flying  is  undoubtedly  the  autumn,  when 
the  crops  are  in.  At  this  time  a  pilot  should  choose  for  preference 
a  stubble  field  which  from  a  height  presents  a  lightish  brown  appear- 
ance. By  doing  this  he  can  1)0  (|uite  certain  that  the  surface  will  be 
smooth  without  ditches  or  mounds,  whereas  the  ordinary  grass  field 
as  often  as  not  abounds  in  the  latter.  Dark  green  fields  ixro  usually 
found  to  be  roots  and  as  such  should  be  avoided  if  better  ground  is 
available.  In  the  winter  rain  may  make  the  stubble  and  root  fields 
very  soft  and  may  make  the  machine  turn  onto  its  nose  on  landing. 
When  they  are  like  this  it  is  very  difficult  to  make  the  machine  rise 
off  the  ground.     Grazing  land  mav  be  identified  l)v  the  feeding 


120  AIR  SERVICE  HANDBOOK. 

cattle.  Should  a  pilot  land  in  growin<i-  wheat  he  should  land  A\-ith 
the  tail  well  do-woi  as  though  the  top  of  the  grain  were  the  ground. 
The  machine  will  then  'pancake"  the  last  foot  or  so  and  unless  it  is 
a  very  light  scout  will  land  without  damage  and  without  turning  over. 
When  a  machine  lands  in  crops  it  should  be  pulled  to  one  side  as 
soon  as  possible  so  as  to  prevent  more  damage  to  the  crops  than  is 
absolutely  necessary. 

Wind. — Navigation  would  be  comparatively  easy  if  wind  did  not 
enter  into  the  question.  It  is  the  more  difficult  to  allow  for  as  it 
varies  both  in  strength  and  direction  at  various  heights.  On  the. 
other  hand  an  intelligent  use  of  the  strength  and  direction  of  the 
wind  may  gi-eatly  aid  the  flight,  by  traveling  high  up  or  low  down  as 
is  most  suitable. 

A  side  wind  will  cause  an  airplane  to  drift — that  is  to  say,  it  will 
have  to  head  up  into  the  wind  a  greater  or  less  extent  in  order  to 
remain  actually  traveling  along  the  course  required.  (Note  that  this 
drift  has  nothing  to  do  with  the  "drift"  of  an  aerofoil.) 

Should  such  a  side  wind  be  blowing  when  a  pilot  is  about  to 
start  on  a  flight  to  a  point  some  distance  away,  it  will  be  quite 
worth  his  while  to  make  a  small  diagram  calculation  on  his  map  to 
ascertain  how  much  he  should  allow  for  it. 

This  can  be  done  in  the  following  manner: 

A  is  the  point  of  departure. 

B  is  the  point  of  destination. 

Join  A  B;  this  will  represent  the  required  course. 

From  A  (point  of  departure)  draw  a  line  down  wind  (i.  e.,  with  a 
southwest  wind  the  line  would  be  northeast  from  A).  Find  out 
the  speed  of  the  wind  from  the  meteorological  map  or  telegram 
(suppose  it  is  20  miles  an  hoiu"). 

Suppose  the  speed  of  the  airplane  is  60  miles  an  hour. 

Lay  off  along  the  wind  line  from  A  two  units  of  measurement;  AC 
will  then  represent  20  miles  an  hour.  Lay  off  along  AB  a  distance 
six  units  in  length.  AD  will  then  represent  60  miles  an  hour. 
Join  CD  and  draw  a  line  AB^  parallel  to  CD  and  equal  to  it  in  length. 

The  compass  course  to  be  steered  will  be  AB^,  and  this  bearing 
can  be  measured  from  the  north  in  the  usual  manner.  Remember 
to  allow  for  variation  of  the  compass.  The  airplane  although  point- 
ing in  a  direction  parallel  to  AB^  will  travel  over  the  ground  along 
the  line  AB,  and  the  distance  AD  measured  in  the  same  unit  of 
distance  will  represent  the  speed  of  the  airplane  relative  to  the 
ground. 

For  the  return  journe>-  a  fresh  diagram  must  be  made.  It  is  not 
sufficient  to  simjily  add  or  subtract  the  error  BAIV  from  the  course 
BA  if  it  were  subtracted  or  added  in  tlic  first  iiistaiire.     The  machine 


AIR  SERVICE  HANDBOOK. 


181 


will  be  in  the  air  a  (lifl'erent  length  of  time  and  tliis  \\-ill  cause  a 
different  angle  of  error. 

A  practical  way  of  finding  the  course  is  to  pass  over  two  points 
on  the  aerodrome  which  are  known  to  be  in  the  same  bearing  as 
that  of  the  distant  point.  By  trial  make  the  macliine  fly  over 
these  two  points  when  flown  on  a  constant  bearing.  Note  tliia 
bearing.  If  the  machine  continues  to  fly  on  this  bearing  it  will 
reach  the  distant  point. 

Always  when  flying  in  a  wind  select  a  point  some  distance  ahead 
over  which  one  ought  to  pass.  This  will  relieve  one  of  the  necessity 
of  continually  looking  at  the  compass  because  the  compass  need 
not  be  checked  till  one  has  reached  the  selected  point  when  another 
point  on  the  required  bearing  should  be  selected. 


■J.,u., 


Fig.  49. 

Time— In  an  airplane  it  is  most  difficult  to  estimate  tim(>.  On 
calm  days  it  seems  to  pass  quickly,  but  on  a  rough  journey  the 
minutes  pass  very  slowly.  Thus  it  often  happens  that  a  pilot  who 
has  not  checked  the  time  of  passing  some  object  expects  to  pass 
the  next  long  before  it  is  really  due.  On  a  reconnaissance  the  time 
should  always  be  kept  in  mind  so  that  one  mil  have  sufficient 
gasoline  in  order  to  bring  one  home. 

Instniments. — The  following  instruments  should  be  fitted  in  an 
airplane  intended  for  cross-country  work: 

A  properly  adjusted  compass. 

A  watch,  fixed  to  the  airplane  by  some  suxtal)le  mounting  which 
])revents  excessive  vibration. 

An  aneroid  mth  adjustable  height  reading. 

An  engine  revolution  indicator.  However  skilled  a  jiilot  may  be 
in  detecting  faulty  running  of  his  engine,  after  a  long  flight  his 


122  AIR  SERVICE  HANDBOOK. 

hearing-  will  not  be  so  good  and  an  indicator  will  assist  him  con- 
siderably. When  flying  over  the  enemies'  lines  it  prevents  a  certain 
amount  of  mental  worry  wondering  if  one's  engine  is  rotating  pi'op- 
erly. 

An  air-speed  indicator.  Tliis  should  always  be  carried  and  is 
especially  necessary  when  a  machine  has  to  fly  at  night  or  through 
clouds.  The  indicator  will  show  changes  in  the  airplane  speed  so 
that  the  pilot  can  tell  when  he  is  climbing  or  going  down.  On  a 
long  flight  one  becomes  tired  and  can  not  tell  how  one  is  flying 
without  the  aid  of  an  instrument. 

An  inclinometer  is  required  for  ascertaining  the  angle  of  flight 
wlien  the  earth  is  not  vi-sible.  For  longitudinal  angles  the  air-speed 
indicator  is  usually  sufficient,  as  by  noticing  whether  the  speed  is 
increasing  or  decreasing  the  pilot  knows  whether  he  is  going  down 
or  u]). 

A  map  case  should  be  ])rovided  wliere  it  can  easily  he  seen  so 
that  the  map  may  be  visible  and  run  no  risk  of  loss  or  damage. 

Lights:  All  the  instruments  should  be  suitably  lighted  in  case 
the  machine  has  to  be  out  after  dark.  This  is  conveniently  done 
by  means  of  small  electric  bulbs  and  dry  cells. 

Gasoline  gauge:  It  is  useful  to  have  a  gasoline  gauge  so  that  the 
amount  left  in  the  tank  can  be  checked  at  once. 
.    An  observer  should  carry  the  following  instruments: 

A  watch  fixed  to  the  outside  of  the  flying  jacket  around  the  arm  or 
leg. 

A  compass,  carried  in  a  similar  manner. 

An  aneroid  is  interesting,  but  not  as  a  rule  absolutely  necessary. 

Maps,  suitable  for  the  job  on  hand  which  should  always  be  carried 
in  a  map  case  or  stuck  to  a  board  and  varnished  in  order  to  prevent 
them  becoming  damaged . 

Message  forms  and  message  bags. 

Report  forms  (these  are  mounted  on  cardboard  in  triplicate  with 
carbon  paper  between  each  sheet). 

Pad  and  pencils  for  making  notes  that  may  be  necessary. 

RULES    OF   THE    AIR. 

1.  Take  off  and  land  directly  into  the  ivind.  Not  only  does  this 
prevent  the  machine  from  turning  over,  but  it  also  does  away  with 
the  risk  of  collision.  The  only  exception  should  be  emergency 
landings. 

2.  Before  starting  see  that  the  section  of  the  field  you  are  going 
to  use  in  making  your  get-away  is  clear  and  that  no  machines  are 
landing  or  gliding  into  this  section  of  the  field.  Locate  position  of 
all  machines  in  the  air.  If  other  machines  precede  you  in  starting, 
allow  them  to  gain  a  distance  of  at  least  half  a  mile  before  followinu, 


AIR   SERVICE   HANDBOOK.  123 

Do  not  follow  (liroctly  in  rear  so  that  the  propeller  wash   will  In- 
avoided. 

3.  Machines  wilh  dead  inotors  have  right  of  way  over  all  others. 

4.  Machines  gliding  into  field  have  right  of  way  over  those  about 
to  leave.  Machine.s  landing  are  often  going  at  a  greater  speed  than 
those  leaving,  so  be  careful  nor  to  misjudge  the  start  and  he  over- 
taken i)y  another  machine. 

'}.  Before  beginning  a  glide  see  that  no  machines  are  underneath 
you.     Those  (lying  beneath  you  have  the  right  of  way. 

<).  In  flight  before  making  a  turn  see  that  no  machines  are  dantrer- 
ously  near  on  your  flanks. 

7.  Unless  there  is  some  urgent  reason,  never  tly  out  of  gliding 
distance  from  a  possible  landing  ground. 

5.  Aircraft  meeting  each  other. — Two  aircraft  meeting  each  other 
end  on  and  thereby  running  the  risk  of  collision  must  always  steer 
out  to  the  right.  They  should  in  addition  to  this  pass  at  a  distance 
of  at  least  100  yai-ds. 

9.  Aircraft  overtaking  each  other.—  .Vny  aircraft  overtaking  another 
aircraft  is  responsible  for  keeping  clear  and  must  not  approach 
within  100  yards  (right  or  left,  above  or  below)  of  the  overtaken 
aircraft.  An  aii'craft  is  said  to  be  overtaking  until  it  ha.s  drawn 
clear  ahead  of  the  overtaken  aircraft.  \Mien  one  of  the  aircraft  is 
an  airship  the  distance  of  100  yards  should  be  increased  to  (iOO  yards. 

10.  Aircraft  approaching  each  other  In  a  cross  direction.-  When  any 
aircraft  are  approaching  each  other  in  cross  directions,  then  the 
aircraft  that  sees  another  aircraft  on  its  right  hand,  forward  quad- 
rant— from  0°  (i.  e.,  straight  ahead)  to  90°  on  the  right  hand — must 
give  way,  and  the  other  aircraft  must  keep  on  its  course  until  both 
are  clear. 

11.  Aircraft  flying  over  an  airdrome  are  bound  by  the  local  rules 
of  the  airdrome  and  an  aircraft  landing  on  an  airdrome  \\-ith  which 
it  is  unfamiliar  must  keep  clear  of  other  machines. 

12.  Night  Ajdng  machines  should  carry  a  green  light  on  the  right- 
hand  wing  tip  and  a  red  light  on  the  left-hand  wing  tip. 

RULE.S    OF   THE    RO.\D-^AII)S    TO    MEMORY. 

If  on  your  right  hand  red  appear, 

It  is  your  duty  to  keep  clear; 

For  he  has  got  the  right  of  way; 

•'Look  out  right  front,"  your  rule  by  day. 

lUit  if  to  left  of  you  is  seen 
An  aviator's  light  of  green, 
There's  not  so  much  for  you  to  do; 
For  gieen  to  left  keeps  clear  of  you. 


124  AIR  SERVICE  HANDBOOK. 

XIV.  NOTES  ON  FLYING. 

GENERAL   INSTRUCTIONS    FOR   PILOTS. 

1.  Before  leaving  the  ground  examine  the  machine  carefully  your- 
self and  then  get  reports  from  both  the  section  chiefs. 

2.  Always  start  against  the  wind  and,  if  possible,  in  a  line  clear 
of  obstacles. 

3.  Leave  with  plenty  of  speed.  Take  a  normal  climb.  Do  not 
climb  your  machine  to  the  limit. 

4.  In  case  the  engine  stops  before  the  machine  has  reached  a  height 
of  600  feet,  land  straight  ahead,  even  if  the  landing  is  bad.  Never 
try  and  turn  down,  wind  so  as  to  get  back  again  onto  the  airdrome. 
This  nearly  always  causes  a  fatal  accident,  even  with  experienced 
pilots. 

5.  In  flying  level  do  not  run  the  motor  full  out  more  than  is  abso- 
lutely necessary;  always  thi'ottle  the  engine,  but  not  so  much  that 
the  machine  is  in  danger  of  losing  flying  speed. 

6.  Land  into  the  wind. 

7.  Land  with  minimum  speed.  Touch  tail  and  wheels  together, 
if  possible. 

8.  Always  taxi  slowly,  and  if  there  is  any  appreciable  wind  let  a 
mechanic  hold  each  wing  tip  so  as  to  prevent  the  risk  of  the  machine 
turning  onto  one  or  the  other  wing  tip. 

9.  Never  leave  a  machine  tail  to  wind.  When  a  machine  is  not 
being  used,  place  it  facing  the  wind  and  tie  the  controls  to  prevent 
them  moving  about  in  the  gusts.  The  controls  should  be  tied  in  such 
a  manner  that  it  is  impossible  for  the  pilot  to  sit  down  without  loosen- 
ing them. 

10.  Always  use  safety  belt.  It  is  much  safer  to  stick  to  the 
machine,  even  if  a  bad  "crash"  is  foreseen. 

11.  In  the  air  certain  machines  have  the  "right  of  way,"  but  this 
does  not  relieve  a  pilot  from  the  necessity  of  keeping  a  good  lookout 
and  from  the  responsibility  of  a  collision. 

12.  Unless  it  is  absolutely  necessary  a  pilot  should  not  start  his 
engine  without  assistance. 

13.  The  pilot  is  responsible  that  the  switch  is  in  the  ' '  off  "  position 
when  the  propeller  is  being  turned  by  hand. 

14.  When  starting  do  not  tiy  to  force  the  machine  off  the  ground. 

15.  Do  not  make  quick  turns  down,  wind  when  close  to  the  ground. 

16.  Remember  that  in  a  quick  turn  the  rudder  puts  the  nose  of  the 
machine  up  oi'  down,  .so  that  if  it  is  found  that  the  machine  is  diving 
put  on  less  rudder  and  pull  ])ack  the  elevator  in  order  to  complete 
the  turn. 


AIR  SERVICE  HANDBOOK.  12S 

17.  To  recover  from  a  ''spiral  dive"  or  "tail  spin"  put  all  controls 
in  a  neutral  position.  The  elevator  control  may  then  be  pushed 
forward  gently.  This  converts  the  spin  inUi  a  "nose  dive,"  out  of 
which  the  machine  can  easily  be  pulled.  The  spin  is  due  to  loss  of 
flying  speed,  so  that  the  essential  thing  to  do  is  to,  first  of  all,  "gain 
speed."  The  use  of  the  rudder  or  ailerons  in  a  case  like  this  merely 
increases  the  drift  and  preA'ents  the  machine  from  gathering  speed. 

18.  Do  not  work  the  controls  roughly,  and  this  especially  applies 
to  the  elevator  controls  when  the  machine  is  di\'ing  at  a  speed. 

19.  Do  not  stand  directly  in  the  plane  of  a  mo^'ing  propeller. 

20.  Tie  all  loose  articles  into  a  machine  so  that  they  can  not  fall 
out,  even  if  it  is  necessary  to  loop  the  machine. 

21.  WTien  coming  to  a  new  airdrome  or  before  landing  on  an 
unknown  ground  always  fly  around  once  or  twice  at  a  few  hundred 
feet  and  make  certain  of  picking  out  a  good  bit  of  ground  for  land- 
ing on. 

22.  When  landing  in  a  restricted  area  do  not  dive  the  machine  in 
order  to  lose  height;  do  proper  S  turns  and  land  slowly. 

23.  Insure  that  all  the  drift  is  off  the  machine  before  landing. 
Land  \\-ith  rudder  neutral. 

24.  In  case  the  engine  fails  when  flying  against  the  wind  it  is 
probably  better  to  make  for  a  landing  ground  down  \nnd  rather  than 
try  to  get  into  a  field  up  wind  with  no  height  to  spare.  It  is  always 
easy  to  kill  height  by  making  a  spiral  or  S  turn. 

25.  \Vhen  gliding,  throttle  down  the  engine  as  much  as  possible 
and  glide  at  the  proper  angle.  There  is  one  angle,  usually  about 
one  in  seven,  which  gives  the  machine  its  best  and  longest  glide. 

26.  There  are  four  types  of  bad  landings  which  it  is  easy  to  make. 
The  first  is  a  "Pancake"  which  results  from  allowing  the  machine 
to  get  into  the  rising  position  when  landing.  In  this  case  there  will 
be  a  perpendicular  bounce  and  on  the  second  bounce  the  landing 
gear  may  break.  To  prevent  this,  open  up  the  engine,  put  the 
machine  in  a  flying  position,  and  then  throttle  down  again  and  land. 
The  second  type  is  the  "Pancake"  which  results  from  bringing  the 
machine  out  of  the  gliding  position  at  a  point  too  far  above  the 
ground,  when  the  machine  \\ill  drop,  due  to  lack  of  speed,  and  may 
break  the  running  gear.  The  third  type  of  bad  landing  results  from 
failure  to  bring  the  machine  out  of  the  glide  at  all,  so  that  it  touches 
the  ground  before  it  is  straightened  up.  This  is  the  most  dangerous 
kind  of  bad  landing.  To  rectify  it,  open  up  the  engine  after  the  first 
bounce  and  put  the  machine  in  the  flying  position,  then  throttle 
down  again  and  land.     The  fourth  kind  of  bad  landing  is  to  land 


126  AIE  SERVICE  HANDBOOK. 

with  drift.  If,  at  the  last  moment,  the  rudder  is  put  over  the 
machine  will  swerve  and  the  side  strain  on  the  landing  gear  may 
pull  off  the  tires  of  the  wheels  or  buckle  them  so  that  the  machine 
may  fall  on  one  wing  tip  or  turn  onto  its  nose. 

27.  Always  test  the  controls  of  the  machine  before  leaWng  the 
ground. 

28.  If  you  become  lost,  do  not  fly  about  aimlessly.  Either  land 
and  ask  your  way  or  else  make  for  some  well-defined  landmark 
which  you  know  or  can  easily  recognize. 

29.  If  one  has  damaged  the  machine  when  landing  away  from  an 
airdrome,  communicate  Avith  headquarters  and  describe  exactly 
what  one  requires  to  make  the  machine  serviceable,  if  possible  call- 
ing each  part  by  its  correct  name.  Do  not  use  the  word  ' '  complete, ' ' 
such  as  "landing  gear  complete,"  but  describe  what  you  want  as 
"wheels,  axle,  etc.  (as  required)." 

30.  When  communicating  your  position  to  headquarters,  describe 
yowc  location  exactly.  Give  the  number  of  the  map  you  are  using 
and  give  your  nearest  large  town  or  some  such  mark,  so  that  your 
location  can  easily  be  found.  Also  give  your  address  and  telephone 
number. 

Cross-country  flying. — Before  starting  on  a  cross-country  flight  be 
sure  that  the  tanks  are  full  of  gasoline  and  oil.  When  taking  over  a 
new  machine  find  out  what  is  the  consumption  of  the  engine  and 
where  the  filling  plugs  are.  AVhile  fljdng  do  not  let  the  gasoline 
get  below  4  gallons,  so  that  in  case  the  first  choice  of  a  landing  ground 
is  bad  you  can  go  up  again  and  choose  another.  When  flying  against 
a  head  wind  do  not  try  to  make  the  airdrome  at  nightfall,  if  there  is 
any  doubt  about  reaching  it.  It  is  much  better  to  land  while  it  is 
light  and  tie  down  the  machine,  rather  than  risk  a  landing  in  the 
darkness. 

Unless  one  is  flying  over  the  lines  one  should  carry  a  small  tool  kit 
for  minor  repairs. 

Care  of  an  airplane  in  the  open. — In  case  a  machine  has  to  be  left 
in  the  open,  steps  must  be  taken  to  prevent  it  blowing  away  and 
becoming  damaged  by  rain  and  dew.  Directly  the  machine 
lands  it  should  be  placed  under  the  lee  of  a  house  or  fence.  The 
pilot  should  take  into  consideration  the  probable  change  in  the 
direction  of  the  wind.  If  the  wind  is  likely  to  blow  strongly,  the 
machine  should  not  be  left  too  near  large  trees,  because  the  branches 
are  liable  to  be  blown  off,  and  falling  on  the  machine  will  damage  it. 
The  controls  of  the  machine  should  be  tied  to  prevent  them  flapping 
about  in  the  wind.  The  elevator  control  should  be  lashed  back, 
so  that  the  wind  will  tend  to  keep  the  tail  on  the  ground.     The 


AIR  SERVICE  HANDBOOK.  127 

machine,  of  course,  should  face  the  wind.  Do  not  turn  the  machine 
tail  to  wind,  because  it  is  not  designed  to  meet  the  wind  in  this 
manner.  If  the  wind  is  likely  to  blow  strongly,  lift  the  tail  by 
l)lacing  boxes  or  trestles  under  the  taliskid.  This  will  give  the 
main  planes  a  negative  angle  and  the  machine  will  tend  to  stay 
on  the  ground  rather  than  to  be  blown  away.  The  wheels  of  the 
machine  should  be  scotched  up  to  the  rear,  or  small  ditches  may 
be  dug  in  which  to  sink  them.  The  landing  gear  should  be  fas. 
tened  to  a  holdfast  in  front  and  the  wings  and  tail  should  be  lashed 
down  to  pickets  in  the  ground.  All  modern  machines  have  small 
rings  on  the  underside  of  the  main  planes  for  this  purpose.  The 
tail  may  be  lixed  by  fastening  the  rope  to  the  tail  skid.  The  pro- 
peller, engine,  and  all  openings  in  the  fuselage  should  be  covered 
by  waterproof  covers,  but  if  these  are  not  available  old  sacks  make 
quite  a  good  i)rotection.  The  most  suitable  form  of  picket  to  use 
is  the  iron  screw  picket,  such  as  is  used  with  many  kinds  of  tents; 
an  ordinary  piece  of  wood  hammered  into  the  ground  is  liable  to 
draw,  especially  if  the  ground  is  wet  and  soft.  Do  not  leave  a  ma- 
chine without  a  guard.  When  a  machine  has  been  left  out  all  night, 
it  sometimes  happens  that  water  collects  inside  the  planes,  the  rud- 
der, etc.  If  this  is  so,  let  it  out  by  pricking  small  holes  in  the 
underneath  of  the  planes.  Most  machines  now  have  small  eyelets 
in  all  trailing  edges  to  prevent  this.  In  case  the  field  is  a  difficult 
one  to  get  out  of,  bear  in  mind  that  in  the  early  morning  the  machine 
will  not  lift  as  well  as  it  ought  to  until  it  becomes  dry.  A  machine 
should  always  be  left  in  the  shade,  because  the  sun's  rays  damage 
a  machine  more  than  wind  and  rain. 

When  machines  are  left  out  in  the  damp,  the  magneto  is  liable  to 
get  damp  inside  and  will  refuse  to  work  in  the  morning.  To  prevent 
this,  wrap  the  magneto  up  in  waste.  This  is  a  very  frequent  cause 
of  delay  in  starting  in  the  mornings. 

Choice  of  airdromes. — Landing  grounds  may  be  permanent  or 
temporary.  A  temporary  ground  need  only  be  good  for  the  time 
of  the  year  for  which  it  is  intended  to  use  it,  but  when  choosing  a 
permanent  ground  the  surface  of  the  soil  and  its  condition  in  rainy 
weather  or  winter  should  be  taken  into  consideration. 

The  following  are  a  guide  in  the  selection  of  landing  ground: 

A.  When  there  is  a  choice  between  two  landing  grounds,  the  one 
in  the  more  open  country  should  be  selected. 

B.  Roads  sufficiently  good  for  heavy  motor  transport  should  lead 
to  the  ground.  A  side  road  leading  to  it  unused  by  ordinary  traffic 
is  also  of  great  use  if  transi)ort  can  be  parked  on  it. 

C.  A  permanent  landing  ground  should  l)e  at  least  500  yards  by 
300  yards  in  size.     A  temporary  ground  for  a  few  airj)Ianes  only 


128 


AIB,  SERVICE  HANDBOOK. 


may  be  as  small  as  200  yards  by  200  yards  if  the  approaches  are  open. 
If  trees  or  telegraph  lines  border  the  ground  a  minimum  of  300 
yards  to  clear  the  obstacle  is  necessary.  This  distance  must  be  in- 
creased if  the  trees  exceed  50  feet  in  height. 

D.  A  good  shape  for  a  landing  ground  is  an  L,  when  the  minimum 
length  of  the  arm  should  not  be  less  than  500  j-ards  and  the  breadth 
300  yards.  An  L-shaped  ground  is  particularly  useful  if  protection 
against  weather  is  naturally  provided  by  the  position  of  trees  or 
houses,  as  in  diagram. 

T-shaped  landing  grounds  are  also  often  to  be  found.  The  length 
of  the  arms  in  this  case  should  not  be  less  than  500  yards  or  the 
breadth  less  than  300  yards. 

E.  Landing  grounds  should  be  as  level  as  possible.  Although 
airplanes  can  rise  from  and  land  on  sloping  ground,  the  wind  will 

often  make  such  landings  diffi- 
cult. An  airplane  rises  and 
lands  up  wind.  It  is  easy  there- 
fore to  rise  when  going  down- 
hill, but  difficult  to  land  under 
the  same  conditions.  Similarly 
it  is  easy  to  land  uphill  but 
difficult  to  rise. 

F.  The  surface  of  the  ground 
should  be  firm  and  level.  The 
best  surface  is  short  grass  or 
stubble.  If  the  surface  is  rough  or  in  ridges  the  landing  gear  is  apt 
to  break  on  landing.  If  the  surface  is  soft  the  airplane  may  tip  up 
and  break  the  propeller,  or  it  may  be  impossible  to  rise  at  all .  Plow 
and  ridge  and  furrow  are  unsuitable. 

G.  Landing  grounds  at  the  bottom  of  hollows  should  be  avoided  if 
possible,  as  they  frequently  become  water-logged  in  wet  weather. 
H.  Landing  grounds  with   telegraph   posts   and  wires  on   their 
boundaries  should  be  avoided. 

Improvevients. — In  the  field  ideal  landing  grounds  are  few  and  far 
between,  but  much  can  be  done  to  improve  them.  This  is  one  of  the 
duties  of  the  Corps  of  Engineers.  Landing  grounds  may  be  improved 
by- 

A.  Rolling  soft  or  rough  ground.  Steam  rollers  are  best  for  rough 
and  hard  surfaces,  but  a  large  stone  or  iron  roller  weighted  with 
pieces  of  timber  is  suitable  for  soft  ground.  If  it  has  been  necessary 
to  select  soft  ground  and  no  rollers  are  available  a  good  deal  may  be 
done  by  a  body  of  men,  such  as  a  company  of  Infantry,  trampling 
down  the  ground. 

B.  Rolling  and  trampling  dry  plow  which  is  not  ridge  and  furrow. 

C.  Filling  up  and  rolling  drains  and  ditches  running  across  the 
ground. 


1 

■ 

G- 

tevMM 

Fig.  50. 


AIR  SERVICE   HANDBOOK.  129 

D.  Cutting  down  trees  and  higli  hedges  when  the  space  lor  landing 
is  less  than  300  yards.  Trees  must  be  felled  so  that  they  fall  away 
from  the  landing  ground. 

E.  Filling  uji  large  ]>ot  lioles  and  rolling.  If  there  is  insutticieut 
time  to  do  this  a  red  or  yellow  flag  or  a  .square  of  red  or  yellow  cloth 
should  be  placed  in  the  center  of  the  pot  hole.  Care  must  be  taken 
that  any  fi!led-in  ground  is  made  firm  and  solid. 

F.  Marking  any  other  dangerous  places,  such  as  ditches  at  the 
edge  of  the  ground. 

G.  Telegraph  posts  and  wires  and  wire  fences  or  iron  railings  must 
be  marked  by  hanging  strips  of  cloth  or  blankets  on  them,  and  the 
Signal  Service  should  be  asked  to  take  down  any  air  lines  which 
might  prove  daiigerous  to  airplanes  and  to  substitute  ground  lines. 

All  work  on  the  landing  ground  should  l)egin  from  the  center  and 
proceed  outward  in  order  that  a  space  for  a  machine  to  land  may  be 
provided  as  quickly  as  i)ossil)le.  With  well  directed  work  very 
unlikely  looking  places  can  be  turned  into  practicable  landing 
grounds  in  a  day. 

It  is  sometimes  advantageous  to  build  paths  for  landing,  radiating 
out  from  a  center,  and  to  put  down  ashes  so  that  these  paths  will  not 
become  muddy  in  wet  weather. 

Working  parties  must  not  leave  their  tools  lying  about  on  the 
ground,  and  when  they  see  a  macliine  about  to  descend  they  must 
at  once  clear  the  ground. 

Methods  of  marking  a  landing  ground  by  day. — Two  strips  of  cloth 
colored  white  and  arranged  in  the  form  of  a  T  should  be  laid  out  ou 
the  airdrome.  The  head  of  the  T  .should  face  directly  into  the  wind, 
thus: 

This  gives  the  direction  in  which  a  pilot  is  to  land.  It  is  to  be 
understood  that  a  machine  landing  in  the  ordinary  manner  so  that  it 
will  stop  running  when  it  reaches  the  head  of  the  T  will  find  good  land- 
ing groiind.  A  machine  should  not  run  over  the  mark,  but  just  to 
one  side. 

The  position  of  the  T  must  be  changed  with  any  change  of  tlie  wind . 

Jfethod  of  marking  landing  ground  by  night. — By  night  landing 
grounds  will  be  marked  with  four  flares  as  under: 

It  is  to  be  understood  that  a  machine  should  touch  the  ground  near 
the  flares  A  or  B  and  that  all  the  ground  between  A  and  CD  is  good 
for  landing  and  free  from  obstacles. 

It  aids  the  pilot  if  a  man  stands  at  each  of  the  flares  so  that  he  is  able 
to  judge  his  height  when  landing. 

Searchlights  may  be  used  to  light  up  landing  grounds.     They 
should  be  placed  near  the  flare  A  and  point  in  the  direction  in  which 
the  machine  will  land.     The  light  should  be  raised  to  about  10  feet. 
46643—18 — —9 


130  AIR  SERVICE  HANDBOOK. 

If  it  is  any  lower  than  this  small  tufts  of  grass,  etc.,  cast  a  shadow 
which  looks  like  a  large  hole  and  this  is  liable  to  interfere  with  the 
pilot  when  he  lands. 

Any  parts  of  the  landing  ground  which  may  cause  damage  should 
be  marked  with  red  lamps. 

The  most  suitable  form  of  flare  is  a  bucket  with  half  a  gallon  of 
gasoline  in  it.  This  will  burn  for  half  an  hour  and  is  visible  from  8 
miles  off  on  a  clear  night  even  when  the  moon  is  half  full. 


wlf?c/ 


_  ^/recrion  of/^>3.chine  Zs.f>c/i no 

Fig.  51. 

On  bright  moonlight  nights  flares  and  lamps  may  be  dispensed 
with  and  the  same  signal  used  as  for  day. 

Aviator's  clothing. — The  clothing  for  an  aviator  must  be  light  and 
Warm  and  ehovild  allow  him  to  make  quick  movements.  The  essen- 
tial for  warmth  is  that  the  aviator  should  be  drj^  when  he  goes  up 
that  is,  he  should  not  walk  about  in  the  wet  grass  and  then  go  up 
into  the  air.  The  clothing  should  not  be  air-tight  and  should  lie 
loose. 

Cap. — The  cap  should  be  of  leather,  lined  with  fur  or  chamois 
leather.     It  should  fit  tight  round  the  face,  so  that  when  the  pilot 


-n' 


Fjg.  52. 


looks  outside  his  machine  the  wind  will  not  be  caught  by  the  sides 
of  the  cap.  If  the  pilot  has  a  comparatively  large  wind  screen  it 
will  cause  the  wind  to  blow  continually  on  the  back  of  his  neck,  so 
that  the  cap  should  come  far  enough  down  so  that  there  is  no  gap 
between  the  cap  and  the  collar.  Sometimes  caps  are  made  so  that 
a  mask  can  be  strapped  on  for  flying  in  cold  weather.  To  prevent 
frostbite  of  exposed  parts  in  cold  weather,  these  parts  should  be 
covered  with  special  frostbite  grease  which  is  an  article  of  store. 

Goggles.- — These  should  fit  both  the  aviator  and  the  cap  so  that  they 
do  not  leave  an  exposed  part  which  would  cause  the  aviator  to  get 
neuralgia.     The  glass  should  be  either  triplex,  or  special  "unbreak- 


AIR   SERVICE   HANDBOOK.  131 

able"  glass.  The  triplex  glass  has  the  disadvantage  that  it  tuts  out 
a  lot  of  the  light  and  is  not  nire  to  use  in  the  early  morning  or  even- 
ing, or  at  night.  It  also  has  the  disadvantage  that  a  comparatively 
light  blow  on  the  outside  will  strip  the  glass  from  the  inside  and 
cause  damage  to  the  eyes.  To  get  the  maximum  efficiency  from  the 
goggles  thoy  should  he  specially  colored.  "Novio"  glass  is  a  good 
type  of  this  colored  glass.  This  glass  is  expensive  and  hard  to  make, 
and  is  quite  different  from  the  cheap,  colored  glas.'^es,  wMch  are  as 
a  rule  worse  than  useless. 

Gloies. — Gloves  should  be  made  of  leather,  lined  with  Jaeger  wool; 
sometimes  silk  gloves  are  worn  also  underneath.  There  is  a  good 
type  of  glove  which  has  a  little  bag  attached  into  which  the  fingers 
can  1)6  slipped  at  the  times  when  nothing  is  happening.  The  gloves 
should  be  loose  so  that  they  slip  on  and  off  easily  and  do  not  stop  the 
circulation.  It  is  an  advantage  to  have  only  the  thumb  and  first 
finger  separate,  the  other  three  fingers  in  a  mitten  without  compart- 
ments so  that  they  will  keep  warm.  Leather  gloves  should  be  kept 
clean.  When  they  become  oily  they  make  the  hands  very  cold. 
If  the  glove  is  a  fur  glove  oil  destroys  the  fur. 

Coat  and  trousers  or  union  suit. — These  should  be  made  of  leather,, 
lined  with  Jaeger  wool  or  fur.  If  fur  is  used  there  should  be  some  sort 
of  inner  lining  to  prevent  the  fur  from  being  torn.  Pockets  in  the 
usual  places  are  useless.  The  most  useful  places  are  diagonally 
across  the  front  over  the  chest  and  at  the  side  of  each  knee.  These 
pockets  can  be  got  at  when  the  aviator  is  strapped  into  the  machine. 
For  this  kind  of  clotliing  there  must  be  some  sort  of  wind-proof 
material  on  the  outside  and  warm  material  on  the  inside.  Leather 
is  best  for  the  outside  material.  A  waterproof  material  on  the  out- 
side causes  moisture  to  collect  on  the  inside,  which  makes  the  aviator 
very  cold.  There  should  be  a  wind  Hap  over  the  opening  of  both 
coat  and  trousers. 

Boots. — Boots  should  be  made  of  leather  and  lined  with  fleece, 
or  at  least  two  pairs  of  thick,  woolen  socks  can  be  worn  inside  instead . 
They  should  be  loose  and  can  be  light.  They  should  be  put  on  just 
before  the  aviator  ascends.  To  keep  them  drj'  while  he  is  walking 
from  the  hangar  to  the  machine  he  should  wear  a  pair  of  rubber 
snow  boots,  which  he  kicks  off  as  he  climbs  into  the  machine. 

lastras. — In  cold  weather  the  aviator  should  take  up  something 
which  he  can  put  in  his  inner  coat  pockets  to  keep  him  warm .  The 
Japanese  charcoal  instra  is  suitable  for  this.  There  is  also  an  electric 
heater  which  can  be  run  off  a  small  generator,  which  is  suitable  for 
this  piu-pose. 

If  an  aviator  has  to  start  his  engine  himself,  he  should  take  off  his 
coat  so  as  not  to  get  too  hot.  If  he  gets  too  hot  before  getting  into 
the  machine  he  will  get  verj-  cold  when  he  gets  high  up. 


132  AIR   SERVICE   HANDBOOK. 

J.t  is  not  a  good  thing  to  take  alcohol  }>efoi-e  going  cm  a  high  flight. 
The  reaction,  by  the  time  one  has  got  into  the  cold,  is  I)ad  and  makes 
one  colder  than  one  would  be  in  the  ordinary  way. 

There  is  a  very  great  difference  between  summer  and  winter  flying 
in  the  matter  of  temperature.     In  the  winter  months  great  care  must 

t 


■t. 


Fig.  53. 


■-W 


be  taken  to  prevent  the  pilot  getting  frostbitten.  Antifreezing 
grease  and  antifreezing  oil  are  used  for  the  face  and  hands.  Elec- 
trically heated  union  suits  have  so  far  proved  the  most  satisfactory 
in  the  way  of  clothing.  Electric  heaters  are  also  required  for  the 
guns.  In  order  to  mitigate  stoppages  and  gun  trouble  generally, 
barrels  and  parts  should  be  kept  as  dry  and  free  from  oil  or  grease 
as  possible.  Covers  and  blinds  will  be  required  for  the 
radiators.  4 

Finding  the  iiortli  point. — -A  well  laid-oiit  compass  •     ^ 

base  is  absolutely  essential  on  every  airdrome, 
and  in  fact  every  squadron  should  have  one  -V  ^^■'■^>, 

for  its  own  use.     The  compasses  on  all  new  •  *^°  C* 

machines  will  I'equire  testing  and  ad-  .■ 

justing   prior    to    any  cross-country  /' 

flying.     It  is  of  the  utmost  impor-  /' 

fance  that  the  compass  in  each  /' 

machine  in  the  squadron  be  /' 

tested  on  the  (;ompass  base         / 
at  regular  intervals,  i.e..  at    ^/ 
least   once  in  every   seven    ^^-rlT'olt. 
days.      Should,  however,         pj^,  ^^^ 
anything    happen    to    the 

machine  that  is  likely  to  affect  the  compass  (such  as  the  litting  in 
of  a  new  engine)  the  compass  must  be  tested  on  the  base  prior  to  any 
cross-country  flight.  There  are  many  occasions  on  which  the  pilot 
will  have  to  depend  upon  the  accuracy  of  his  compass  for  the  safety 
of  himself  and  his  machine.  The  north  point  can  l)e  found  by  night 
or  by  day  as  follows: 


AIR   SERVICE   HANDBOOK.  13S 

/)'//  iii(jhl.-  In  tiie  Xortliciii  lli'iuispln'ic  it  will  he  noticiMl  thai  all 
the  stars  re\'olve  around  a  single  t)ni',  wliich  is  called  the  North 
Star.  This  star  can  be  found  easily  with  reference  to  the  constel- 
lation called  the  Great  Bear.  The  Great  Bear  consists  of  seven 
stars,  arranged  as  shown  above.  One  can  imagine  them  to  repre- 
sent a  saucepan.  "The  handle  of  the  saucejjan  points  to  the  left  when 
the  constellation  is  below  the  North  Star  and  to  the  right  wlien  it  is 
above.  The  part  of  the  saucepan  opposite  the  handle  points  directlj- 
toward  the  North  Star,  and  the  two  stars  which  form  this  are  called 
the  pointers.  The  North  Star  itself  makes  a  small  circle  around 
the  North  Pole.  The  actual  pole  may  be  found  by  drawang  a  line 
from  the  Pole  Star  to  the  second  star  in  the  handle  of  the  sauce- 
pan, the  one  called  p.  The  North  Pole  lies  on  this  line  and  2° 
(as  measured  from  tlie  (^arth)  away  from  the  Pole  Star.  The  con- 
stellation wliich  is  on  the  opposite  of  tlie  North  Star  to  tlie  (ireat 


♦  >io-|-;. 


Bear  is  called  Cassiopea.  This  looks  like  a  big  W  and  the  North 
Star  is  the  first  bright  star  right  above  this  letter. 

In  the  Southern  Hemisphere  the  North  Star  is  not  visible,  but  the 
South  Pole  may  be  found  with  i-eference  to  the  Southern  Cross. 
The  Southern  Cross  consists  of  four  bright  stars,  which  can  be  imag- 
ined to  form  the  ends  of  the  cross.  There  is  a  less  bright  star  close 
to  one  of  these  corners.  There  is  another  constellation  not  very 
far  away,  which  looks  something  like  this,  but  it  is  not  nearly  so 
symmetrical  and  should  not  be  mistaken  for  the  projier  cross.  In 
order  to  find  the  South  Pole  draw  a  line  through  the  long  arm  of 
the  (TOSS,  divide  the  part  between  the  two  stars  into  three.  Then 
measure,  awa>'  from  the  longer  part  of  this  arm,  a  distance  corre- 
sponding to  nine  n\  these  divisions.  This  point  ^\^ll  be  the  South 
Pole. 

By  (lay.  Plant  a  stick  in  I  lie  giound,  pointing  api)roxiniately 
toward  the  north.  Tie  a  ])lumb  l)ob  to  the  top  of  the  stick  and 
allow  it  to  touch  the  ground.     From  this  point  draw  a  (irde  on  the 


134  AIR   SERVICE   HANDBOOK. 

ground  with  any  convenient  radius.  Note  the  two  points  where 
the  shadow  of  the  top  of  the  stick  touches  the  circle.  There  will 
be  one  place  in  the  morning  and  one  place  in  the  afternoon.  Join 
these  two  points  to  the  center  of  the  circle.  The  North  Pole  will 
be  found  by  halving  this  angle. 

For  c-hecking  the  comimss  it  should  be  remembered  that  these 
points  are  true  north.  The  compass  points  on  the  ground  should 
therefore  be  laid  out  after  having  made  the  suitable  correction  for 
the  variation  of  the  compass  on  the  airdrome. 

For  steering  by  the  stars  a  knowledge  of  the  constellations  is  not 
absolutely  necessary,  but  it  is  very  hard  to  keep  a  course  on  a  single 
star  without  this  knowledge. 

XV.  METEOROLOGY. 

Introductory  remarks. — Although  in  late  years  great  advances  have 
been  made  in  the  •  knowledge  of  the  conditions  and  changes  taking 
place  in  the  atmosphere,  yet  a  very  large  number  of  cpiestions  of 
great  interest  from  an  aeronautical  standpoint  still  remain  unan- 
swered . 

Further,  changes  taking  place  in  the  atmosphere  ai-e  very  com- 
l)licated,  so  that  the  problem  of  forecasting  weather  in  detail  is  a 
matter  of  the  greatest  difficulty. 

Importunately  the  wind,  whi<'h  is  one  of  the  most  important  factors 
to  the  aviator,  is  the  most  amenable  to  simple,  physical    laws. 

The  composition  of  the  atmosphere. — The  atmosphere  is  composed 
of  nitrogen,  oxygen,  carbonic  acid  gas,  water  vapor,  dust,  and  cer- 
tain other  gases  in  small  quantities.  All  these  are  mixed  together 
and  are  not  joined  chemically.  The  dust  in  the  atmosphere  furnishes 
solid  ]iarticles,  around  which  the  water  vapor  condenses  to  form  fog 
or  rain  and  also  gives  the  colors  of  the  sky  and  causes  twilight. 
Over  the  trenches  there  is  a  considerable  amoimt  of  dust  caused  by 
the  exploding  of  shells  and  guns.  Not  only  does  this  help  clouds 
to  form,  but  it  affects  the  visibility  of  different  points  and  makes 
it  hard  to  take  clear  photographs  near  the  lines. 

The  atmosphere  probably  extends  50  to  200  miles  above  the  sur- 
face of  the  earth .  That  part  of  it  which  is  dense  enough  or  contains 
enough  oxygen  to  support  life  is  limited  to  about  30,000  feet.  Ma- 
chines which  fly  normally  abovc^  15,000  feet  should  carry  oxygen, 
otherwise  the  li(>arts  of  the  crew  of  the  airplane  are  liable  to  be 
strained. 

The  weight  of  a  column  of  air  at  normal  temperatiu'e  and  at  sea 
level  is  about  L5  pounds  per  square  inch,  which  corresponds  to  the 
weight  of  a  column  of  mercury  30  inches  high.  The  air  at  approxi- 
mately 20,000  feet  is  half  as  dense  as  it  is  at  sea  level.  The  density 
of  the  air  affects  the  efficiency  of  the  airplane  engine  considerably- 


AIR   SERVICE   HANDBOOK.  136 

Atmospheric  pressure. — The  pressure  of  the  air — which  will  be  seen 
later  to  be  a  variable  quantity  and  its  changes  to  be  of  gi-eat  use  in 
forecasting  weather — is  clue  to  the  weight  of  air  above  the  place 
where  the  pressure  is  exerted.  It  will  be  readily  seen  that  the 
longer  the  vertical  cohimn  of  air,  and  the  greater  the  density  of  the 
air,  the  greater  also  will  be  the  pressure  exerted  at  the  bottom  of  the 
column.  Hence  at  two  places,  one  above  the  other,  the  pressure  at 
the  lower  place  will  be  equal  to  the  pressure  at  the  upper  plus  the 
pressure  due  to  the  weight  of  tlie  air  between  the  two.  This  differ- 
ence in  pressure  will  not  be  constant,  but  will  depend  on  the  density 
of  the  air,  which  in  turn  varies  with  the  temperature  and  pressure. 

The  formula  connecting  the  difference  of  pressure  at  two  places 
has  been  given  under  ''Barometer." 

The  temperature  of  the  air  generally  falls  off  with  height,  but  near 
the  surface  of  the  ground  the  rate  of  decrease  is  often  far  from  con- 
stant, and  it  is  not  uncommon  to  find  a  warmer  layer  of  air  above  a 
colder  one.  The  average  rate  of  decrease  is  about  1°  F.  for  every  300 
feet.  Above  5,000  or  6,000  feet  the  rate  of  decrease  of  temperature 
becomes  nearly  constant  at  1°  F.  for  every  300  feet.  At  very  grea 
heights  (over  30,000  feet)  the  temperature  ceases  to  fall  with  height 
and  may  sometimes  rise  again.  This  region,  however,  is  at  present 
aboA'e  the  height  practicable  for  flying. 

If  readings  of  the  atmospheric  pressure  as  measured  by  a  barometer 
are  taken  at  the  same  time  at  a  number  of  diffei-ent  places  situated 
over  a  wide  area,  and  are  then  plotted  on  a  map  against  each  station 
the  readings  will  be  found  to  he  arranged  in  some  order,  ('ertain 
areas  will  have  low  pressure  and  others  will  have  higli  pressure. 

On  any  topographical  map  lines  or  contours  are  drawn  showing  the 
heights  of  the  ground  equally  on  the  pressure  map  it  is  possible  to 
draw  similar  contours  showing  the  heights  of  the  barometer.  As  all 
places  on  any  one  contoiir  are  the  same  height  so  all  places  on  any 
line  on  the  pressure  map  will  have  the  same  atmospheric  pressure. 
These  lines  are  called  "Isobars." 

The  isobars  are  generally  drawn  for  each  tenth  of  an  inch  of  mer- 
cury, i.  e.,  the  difference  of  pressure  between  two  places  on  two  con- 
secutive isobars  will  be  one-tenth  of  an  inch  of  mercury.  In  regions 
where  there  is  a  large  difference  of  pressure  between  places  not  far 
apart  the  isobars  will  necessarily  be  close  together,  just  as  on  a  map, 
where  the  slope  of  the  ground  is  steep  the  contours  will  be  close 
together. 

The  rate  at  which  pressure  changes  from  place  to  place  is  known 
as  the  "Pressure  gradient."  When  the  pressure  is  changing  rapidly 
the  "Gradient"'  is  said  to  be  "steep." 


136 


AIR   SERVICE   HANDBOOK. 


On  weather  maps  there  are  other  lines  marked  in  red  which  show 
lines  of  equal  temperatures.  These  lines  are  marked  for  differences 
of  20°  F.  and  are  called  '"isotherms." 

The  wind  and  its  connection  vith  atmospheric  pressure. — When  one 
part  of  the  country  is  under  high  pressure  and  another  under  low 
pressure,  it  might  be  supposed,  at  first,  that  aii-  would  be  forced  out 
of  the  region  of  high  pressure  toward  that  of  the  lower  pressure,  and 
that  winds  would  be  found  everywhere  blowing  straight  outward 
from  the  high  pressures  and  straight  inward  toward  the  low  pres- 
sures. Reference  to  any  weather  chart  will,  however,  show  that 
this  is  not  what  happens.  The  winds  blow  in  a  direction  which  is 
more  nearly  parallel  to  the  isobars  than  at  right  angles  to  them.  The 
explanation  of  this  phenomenon  is  found  in  two  facts: 

A.  The  earth  is  revolving  about  its  axis.  This  causes  all  winds 
in  the  Northern  Hemisphere  to  be  deflected  to  the  right  of  the  path 


which  they  would  follow  if  they  were  affected  only  by  the  " '  Pressure 
gradient,"  and  tends  to  make  them  blow  parallel  to  the  isobars; 
similarly  winds  in  the  Southern  Hemisphere  are  deflected  to  the 
left  of  this  path. 

B.  Friction  between  the  air  and  the  surface  of  the  ground  tends  to 
lessen  the  deflection  of  the  winds  caused  by  the  rotation  of  the  earth. 

The  result  of  these  phenomena  is  that  the  winds  at  the  surface  blow 
round  the  centers  of  low  pressure  in  incurving  spirals  in  an  anti- 
clockwise direction  (in  the  Northern  Hemisphere)  and  in  outcurving 
spirals  in  a  clockwise  direction  round  the  center  of  high  pressure. 

If  it  were  possible  to  eliminate  sin-face  friction,  the  velocity  of 
the  wind  could  l)e  calculated  theoretically  froni  the  ''Pressure 
gradient."  An  imaginary  wind  having  this  theoretical  velocity 
and  direction  is  called  the  '"Gradient  wind."  and  its  velocity  and 
direction  are  known  as  the  ''(iradient  velocity"  and  the  '"Gradient 
direction . ' ' 

At  a  height  of  I  .OOO  to  2,000  feet  al)Ove  the  .surface  of  the  ground 
the  effect  of  surface  friction  is  very  small  and  the  actual  wind  has 
A'erv  nearlv  the  "(Jradient  velocity  and  direction." 


AIR   SERVICE   HANDBOOK.  187 

Table  —  shows  the  ■"Gradient  velocity"  for  different  values  of 
the  "Pressure gradient. " 

The  strength  of  the  wind  is  generally  expressed  in  terms  of  its 
velocity  in  miles  per  hour.  For  some  purposes  it  is  more  convenient 
to  use  a  rougher  classification  and  to  divide  all  winds  from  calm  to 
a  hurricane  into  12  groups,  denoting  the  strength  of  the  wind  ])y 
the  numhor  of  the  group  into  which  it  would  fall.  As  this  system 
is  duo  to  Admiral  Beaufort,  it  is  known  as  the  "•Beaufort  scale." 
The  strength  of  the  wind  may  also  be  given  in  terms  of  the  pressure 
exerted  by  it.  say.  on  a  flat  plat(v  This  pressure  varies  as  the 
square  of  the  velocity. 

Table  — •  gives  the  relation  between  the  velocity  in  miles  per 
hour,  the  Beaufort  number,  and  the  pressure  exerted  on  a  Hat 
plate  in  pounds  per  square  foot. 

"Gradient  direction"  is  along  the  isol)ars  with  the  low  pre.saure 
on  the  left  hand. 


Gusthic.'i.s  of  the  irinrl. — It  is  found  that  the  velocity  of  the  wind 
does  not  remain  constant.  It  is  continually  changing,  and  as  a 
result  is  always  rather  gusty.  The  gustiness  varies  with  different 
places  and  with  different  diiections  of  the  wind.  The  difference 
between  the  average  maximum  velocity  attained  in  the  gusts  and 
the  average  minimum  velocity  attained  in  the  intermediate  lull 
is  known  as  the  ""  Fluctuation"  of  the  wind. 

The  ratio  of  the  ""Fluctuation" '  to  the  mean  velocity  of  the  wind 
is  called  its  'Gustiness,"'  and  this  is  found  to  1)e  roughly  constant 
for  any  one  direction  at  any  ))lace.  whatever  may  l)e  the  mean 
A'elocity  of  the  wind. 

Effect  of  the  irind  sirikiny  obstacles. — If  the  ol)Stacles  are  low  hills, 
such  as  are  found  on  the  plains  and  in  rolling  country,  the  wind  may 
approximately  follow  the  surface.  If  the  obstacle  is  abrupt,  such 
as  the  side  of  a  house  or  a  vertical  cliff,  the  air  striking  the  oljstacle 
will  be  deflected  upwards  and  will  not  touch  the  surface  of  the 
earth  again  for  some  distance.  Thus,  in  the  lee  of  a  tall  building 
there  is  often  a  calm  area  or  the  wind  may  Ix-  l)lowing  in  the  opposite 
direction  to  the  wind  tip  above. 


138  AIR   SERVICE   HANDBOOK. 

This  is  especially  noticeable  in  i)laces  like  (iil)raltar  when  the 
east  Avind  is  lilowing.  A  iierson  standing  on  the  edge  of  the  cliff 
may  l)e  in  absolutely  calm  air  but  on  pushing  out  his  arm  he  will 
feel  the  wind  l^lowing  vertical  u])ward  at  a  considerable  speed. 
The  wind  blowing  over  the  hangars  on  the  airdrome  has  caused  a 
number  of  accidents  because  pilots  forget  that  there  is  a  down  current 
which  is  often  strong  enough  to  prevent  the  machine  clearing  the 
hangars. 

In  the  old  days  machines  which  had  practically  no  reserve  of 
power  and  which  could  only  fly  level  when  near  the  ground  found 
it  very  difficult  to  fly  in  windy  weather.  A  machine  on  passing 
over  a  wood,  for  instance,  might  be  caught  in  the  down  current 
on  the  lee  side  and  in  some  cases  machines  had  not  enough  power 
to  prevent  themselves  hitting  the  ground.  In  the  summer  in  sunny 
weather  it  was  quite  dangerous  to  cross  a  river  because  the  down 
current  would  suck  the  machine  practically  into  the  water.  The 
pilot  at  the  present  time  has  nothing  to  fear  from  these  causes. 
Machines  are  so  highly  powered  that  they  liave  a  much  greater 
reserve  of  power  than  they  will  be  called  upon  to  use  when  affected 
by  the  changes  of  velocity  in  the  air.  At  the  present  time  if  the 
pilot  does  not  go  merely  seeking  danger,  wind  only  means  that  the 
machine  will  take  longer  when  flying  upwind  and  will  take  a  shorter 
time  to  reach  a  place  when  flying  down  wind.  The  only  thing 
which  sto|)s  war  flying  is  low  fog  which  prevents  a  pilot  seeing 
where  he  lias  got  to  and  will  prevent  him  from  flnding  his  airdrome 
when  he  returns.  If  a  pilot  obeys  the  ordinary  rules,  such  as  getting 
off  and  landing  directly  into  the  wind,  he  has  nothing  to  fear  from 
such  things  that  used  to  l>e  called  ' '  holes  in  the  air.  "  ' "  air  pockets, ' ' 
etc . .  and  from  such  things  as  are  called  cyclones,  aerial  cataracts,  etc. 

FORECASTING. 

From  the  foregoing  it  has  been  seen  that  when  baxometric  pres- 
sures are  plotted  on  a  map  they  are  arranged  according  to  some 
order.  It  is  now  necessary  to  consider  certain  typical  cases  of 
pressure  distribution. 

A.   The  cyclone. — 

This  type  consists  of  a  center  of  low  pressure  from  which  the 
pressure  rises  on  all  sides.  The  isobars  are  roughly  circular  about 
the  center  of  low  pressure.  The  winds  blow  in  an  anticlockwise 
direction  round  the  center  (clockwise  in  the  Southern  Hemisphere). 

The  different  parts  of  a  cyclone  have  each  their  own  type  of 
weather  of  which  the  following  description  may  be  given. 

The  temperature  is  always  higher  in  front  than  in  rear,  the  warm 
air  in  front  having  a  peculiar  close,  mugg>'  character,  quite  inde- 


AIR   SERVICE   HANDBOOK. 


139 


pendent  of  the  actual  height  of  the  thermometer.  The  cold  air  in 
the  rear  on  the  contrary  has  a  peculiarly  exhilarating  feeling,  also 
quite  independent  of  the  thermometer. 

The  force  of  the  wind  depends  almost  entirely  on  the  '"Gradient." 
In  the  center  it  is  dead  calm  and  the  steepest  'Gradients'''  are 
usually  found  at  some  distance  from  center. 

The  relative  steepness  of  the  "Gradients^'  measures  the  intensity 
of  cyclones. 

If  two  lines  are  drawn  thiough  the  center  of  the  cyclone,  one  in 
the  direction  parallel  to  that  of  its  motion  and  another  at  right 
angles  to  this  direction,  the  cyclone  will  be  divided  into  four  quad- 
rants, each  of  which  has  its  pecidiar  tj-pe  of  weather.     The  line  at 


yVin<^y/S  C?iancf/r,a  />/K>V 


/"o  Sr-^ 


Fig.  5s. 

right  angles  to  the  direction  of  motion  is  known  as  the  line  of  the 
"trough." 

The  broadest  feature  of  the  weather  in  an  average  cyclone  con- 
sists of  an  area  of  rain  near  the  center  surrounded  by  a  ring  of  cloud, 
but  both  rain  and  cloud  extend  farther  to  the  front  than  to  the  rear, 
of  the  center.  When,  however,  examining  the  nature  of  the  cloud 
and  rain  as  well  as  the  general  appearance  of  the  sky.  it  is  found 
that  the  cyclone  is  di\'ided  into  two  well-marked  halves  by  the 
line  of  the  "trough."  The  front  may  be  further  divided  into  right 
or  southeast,  and  left  or  northeast  fronts  which,  though  they  have 
much  in  common,  are  sufficiently  different  to  be  classified  separately. 

Coming  now  to  more  minute  detail,  in  the  left  or  northeast  front 
when  the  steepest  "Gradients"  are  somewhere  south  of  the  center, 
the  first  symptoms  of  the  approach  of  a  cyclone  are  a  halo,  with  a 
gradual  darkening  of  the  sky  until  it  becomes  quite  overcast  with- 
out any  appearance  of  the  formation  of  true  clouds;  or  else  liglit 


140  AIR   SERVICE   HANDBOOK. 

wisps  or  barred  stripes  of  ciri'us  moving  sideways,  apj^ear  in  the 
l>lue  sky.  and  gradually  soften  into  a  uniform  black  sky  of  a  strato- 
'umulus  type;  near  the  center  light  ill-defined  showers  fall  from  a 
uniformly  lilack  sky,  the  wind  from  some  point  between  southeast 
and  northeast  blows  uneasilj^,  and  though  the  aii'  is  cold  and  cliilly 
there  is  an  opjjressive  feeling  about  it.  These  appearances  con- 
tinue until  the  barometer  commences  to  rise,  when  the  character  of 
the  weather  at  once  begins  to  alter.  In  a  cyclone,  when  the  steepest 
"Gradients"  are  somewhere  to  the  north  and  east  of  the  center,  the 
general  character  of  the  weather  is  the  same  as  above  described,  but 
much  more  intense.  The  wind  rising  at  times  to  a  heavy  gale,  and 
the  ill-defined  showers  developing  into  violent  squalls. 

In  the  right  or  southeast  front,  when  the  steepest  "Gradients" 
are  to  some  point  south  of  the  center,  which  are  the  commonest 
cases,  the  first  symptoms  are  likewise  a  gradual  darkening  of  the 
sky  into  the  well-known  pale  or  watery  sky,  with  muggy,  oppressive 
air;  or  else,  as  in  the  northeast  front,  msps  of  cirrus  first  appear  in 
the  blue  sky  which  gradually  becomes  hea\'ier  and  softer  until  the 
sky  is  uniformly  overcast  with  a  strato-cumulus  type  of  cloud. 
Near  the  center  rain  usually  in  the  form  of  a  drizzle  sets  in  and  the 
wind  from  some  point  between  southeast  and  southwest,  vary  in 
force  according  to  the  steepness  of  the  "Gradients,"  drives  the 
cloud  and  rain  before  it. 

In  winter  snow  takes  the  ])lace  of  rain  and  in  the  autumn  the 
northeast  wind  brings  with  it  little  flurries  of  snow. 

The  line  of  the  "trough"  marks  the  line  of  heavy  showers  or 
squalls,  especially  tlie  portion  on  the  southern  side  of  the  center. 

The  general  character  of  the  west  of  the  depression  is  a  cool, 
exhilarating  feeling  in  the  air,  with  a  high,  hard  sky  of  which  the 
tendency  is  always  to  break  into  firm,  detached  masses  of  cloud. 
The  rain  which  occurs  near  the  center  is  usually  in  cold,  hard, 
brisk  showers  or  hard  squall*?,  and  the  general  look  of  the  weathsr 
presents  a  marked  contrast  to  the  dirty  appearance  of  the  weather 
wluch  characterizes  the  whole  front  part  of  a  cyclone.  Farther 
from  the  center  showers  or  squalls  are  replaced  bj^  simply  detached 
masses  of  cloud,  and  finally  these  disapjiear  leaving  a  blue  sky- 
The  wind  fi'om  .some  point  between  west  and  north  blows  gustily. 

The  whole  of  the  rear  of  a  cyclone  partakes  of  this  general  char- 
acter, but  the  change  of  weather  along  the  north  of  the  cyclone  is 
not  nearly  so  well  marked  as  along  the  southern  portion. 

The  motion  of  cyclones  is  as  a  rule  from  west  to  east,  the  general 
direction  being  about  west-southwest  to  east-northeast.  Occa- 
sionallv  thcv  move  north  or  south,  but  seldom  from  east  to  west. 


AIR   SERVICE   HANDBOOK.  141 

They  may  also  at  times  remain  stationary  for  a  (.lay or  two,  but  this 
is  rare. 

The  followinj^  indications  are  printed  on  every  weather  map: 

When  the  wind  sets  in  from  points  between  south  and  southeast 
and  the  barometer  falls  steadily  a  storm  is  approaching  from  the 
west  or  northwest,  and  the  center  will  pass  near  or  north  of  the 
observer  within  12  or  24  hours,  with  wind  shifting  to  northwest  by 
way  of  southwest  and  west. 

WTien  the  wind  sets  in  from  points  between  east  and  northeast 
and  the  barometer  falls  steadily  a  storm  is  approaching  from  the 
south  or  southwest,  and  its  center  will  pass  near  or  to  the  south  or 
east  of  the  observer  ^vithin  12  or  24  hours,  with  wind  shifting  to 
northwest  by  way  of  north. 

The  rapidity  of  the  storm's  approach  and  its  intensity  will  be  in- 
dicated by  the  rate  and  the  amount  of  the  fall  in  the  barometer. 

B.  The  antiq/clone. — The  distribution  of  pressure  in  an  anticyclone 
is  almost  the  reverse  of  that  in  a  cyclone.  It  consists  of  a  central 
area  of  high  pressure  from  which  the  pressure  gradually  decreases  on 
all  sides.  The  "'Gradients"  are  generally  very  slight  so  that  the 
winds,  which  blow  around  the  center  in  a  clockwise  direction,  are 
very  light.  Unlike  a  cyclone,  no  very  definite  description  of  the 
weather  can  be  given;  in  fact,  almost  any  tjT)e  of  weather  may  be 
found  in  an  anticyclone  except  strong  winds;  heav^'  rain  is  also  in- 
frequent. On  the  whole,  the  weather  is  tine,  but  in  ^vinter  periods 
of  dull,  cloudy  weather  often  accompany  an  anticyclone.  On  the 
other  hand,  days  with  cloudless  skies,  and  keen,  frosty  weather  in 
winter  or  hot  weather  in  summer,  frequently  occur  in  anticyclones . 
Unlike  cyclones,  they  have  no  general  direction  of  motion,  but  move 
very  slowly  and  in  any  direction.  They  frequently  remain  sta- 
tionary for  several  days  together.  In  the  colder  months  anticyclones 
are  very  frequently  accompanied  by  fog. 

C.  Secondary  (/epnssions. — On  the  outside  of  a  cyclone  irregu- 
larities in  the  isobars  frequently  occur.  These  may  be  mere  kinks 
in  the  isobars,  or  they  may  be  well  marked  and  have  an  independent 
area  of  low  pressure.  A  very  common  form  is  for  the  isobars  to  have 
approximately  the  shape  of  a  V,  such  cases  being  known  as  V  de- 
pressions. The  secondaries  travel  around  the  main  depression  in 
the  same  direction  as  do  the  winds,  viz,  anticlockwise. 

The  weather  in  these  secondaries  varies  according  to  whether  they 
are  well  marked  or  not.  ^Vhere  only  a  small  kink  occurs,  only 
cloudy  skies  and  temporary  rain  may  be  produced,  but  if  they  are 
well  marked  and  the  "Gradients"  are  steep  the  winds  may  become 
very  strong  and  the  weather  very  bad. 


142  AIR   SERVICE   HANDBOOK. 

In  a  secondary  depression  of  average  intensity  the  sequence  of 
weather  is  as  follows : 

As  the  secondary  approaches,  the  weather  is  similar  to  that  in  the 
right  front  of  a  cyclone;  as  the  secondary  passes,  the  barometer  sud- 
denly begins  to  rise  and  there  is  frequently  a  heavy  squall,  as  in 
the  "trough"  of  a  cyclone,  and  the  wind  suddenly  veers  to  a  more 
northerly  quarter. 

On  the  side  away  from  the  center  of  the  main  cyclone  the  winds 
are  generally  very  strong,  but  between  the  secondary  and  the  main 
depression  they  are  light.  In  the  rear  of  a  secondary  the  weather 
is  similar  to  that  in  the  rear  of  a  cyclone. 

D.  The  wedge. — It  frequently  occurs  that  a  series  of  cyclones  pass 
across  the  country  in  a  continuous  succession.  Between  two  of  these 
cyclones  the  isobars  will  be  roughly  V-shaped,  but  in  this  case  with 
the  highest  pressure  within  the  V.  These  are  times  of  brilliantly 
tine  weather,  cloudless  skies,  and  clear  atmosphere,  but  as  another 
cyclone  is  approaching  they  last  only  a  short  time. 

E.  Line  squalls. — It  has  been  said  that  as  the  "  trough  "of  a  cyclone 
passes  there  is  frequently  a  heavy  squall.  This  is  generally  of  the 
type  known  as  a  "Une  squall."  Such  a  squall  stretches  in  a  Line 
for  a  long  distance  across  the  country  and  may  be  as  much  as  500 
miles  in  length .  The  squall  moves  in  a  direction  approximately  at 
right  angles  to  its  length  with  a  velocity  of  between  30  and  50  miles 
an  hour.  The  breadth  of  the  squall  is  usually  narrow,  so  that  it  does 
not  last  long — generally  from  half  an  hour  to  two  hours.  The  squall 
gives  no  sign  of  its  approach,  except  that  if  the  sky  is  fairly  cloud- 
less a  long  Une  of  well-marked  cumulus  may  be  seen  in  the  distance, 
gradually  coming  nearer.  As  the  squall  reaches  the  observer  the 
wind  suddenly  increases  (or  occasionally  increases  sUghtly  and  then 
suddenly  decreases)  and  at  the  same  time  the  direction  suddenly 
changes  to  a  more  northerly  quarter. 

The  barometer  generally  shows  a  small,  sudden  rise,  and  the  tem- 
perature always  suddenly  falls.  Heavy  rain  and  frequent  hail, 
with  sometimes  thunder,  set  in  at  once.  The  whole  squall  is  of  a 
violent  nature  and  it  may  do  considerable  damage.  This  phenom- 
enon seems  to  be  caused  by  the  sudden  inrush  of  a  cold  current  of 
air  from  some  northerly  quarter,  which  forces  the  warmer  air  in 
front  of  it  to  ascend.  As  these  squalls  give  no  warning  of  their  ap- 
proach, and  as  they  are  very  violent,  they  are  of  a  particularly  dan- 
gerous nature.  They  are  to  be  expected  when  the  "trough"  of  a 
depression  passes,  and  especially  in  a  V,  or  secondary.  After  one 
squall  has  passed,  others  frequently  follow  in  the  next  few  hours. 
These  "line  squalls"  may  also  occur  at  times  in  conditions  that 


AIR  SERVICE  HANDBOOK.  143 

would  be  expected  to  give  only  a  moderate  westerly  or  southwesterly 
wind.  An  observer  can  only  forecast  these  phenomena  when  he  is 
in  possession  of  information  that  a  squall  has  passed  certain  points 
and  is  traveling  in  his  direction. 

F.  Fog. — True  fog  (clouds  on  the  surface  of  the  earth  are  not  true 
fog)  on  land  is  only  formed  when  there  is  little  wind  and  when  the 
sky  is  cloudless.  During  the  day  the  air  is  warm  and  takes  up  water 
vapor.  On  a  calm,  cloudless  evening  the  ground  is  cooled  by  out- 
ward radiation  and  the  aix-  near  the  ground  is  also  cooled.  This 
cold  air  being  heavier  flows  down  the  sides  of  hills  and  mixes  with 
the  warm,  moist  air  over  the  valleys,  which  is  thereby  cooled.  The 
cooling  so  produced  may  be  sufficient  to  cause  some  of  the  water 
vapor  to  condense,  and  fog  is  formed.  If  there  is  much  wind  the 
air  is  kept  stirred  up  and  no  cold  air  is  formed.  If,  on  the  other  hand, 
the  sky  is  cloudy,  the  ground  is  not  cooled  by  radiation,  so  that  in 
this  case  no  fog  is  formed. 

G.  Conditions  of  the  atmosphere  affectinq  aviation. — The  available 
knowledge  of  the  upper  air  is  still  rather  small,  but  some  information 
has  been  obtained  which  is  of  use  to  aviators.  If  it  is  required  to 
ascertain  what  the  wind  is  at  a  height  above  the  ground,  there  are 
several  methods  by  which  this  information  may  be  obtained. 

First,  by  sending  up  a  small  balloon  which  drifts  along  with  the 
velocity  of  the  wind  at  the  height  it  has  reached.  The  balloon 
is  observed  by  theodolites,  and  the  velocity  and  dii-ection  of  the 
wind  at  any  height  can  be  calculated  accurately.  This,  however, 
is  an  elaborate  method  and  takes  considerable  time. 

Secondly,  some  information  may  be  obtained  by  observing  the 
motion  of  the  clouds.  From  these  the  direction  of  the  wind  at  their 
level  may  be  accurately  gauged.  It  must,  however,  be  remembered 
that  the  height  of  the  clouds  can  not  definitely  be  fixed  without 
suitable  instruments,  and  therefore  the  velocity  is  only  very  approxi- 
naate.  Nevertheless,  a  rough  idea  may  be  obtained  by  noting 
■whether  the  clouds  are  moving  ([uickly  or  slowly.  The  lower  the 
clouds  are  the  faster  they  "appear"  to  move. 

Thirdly,  an  estimate  of  the  upper  w.ind  may  be  obtained  from  a 
daily  weather  map  by  calculating  the  "Gradient"  wind.  At  a 
height  of  a  thousand  feet  or  more  the  "Gradient"  wind  is  found  to 
agree  very  well  with  the  winds  at  those  heights  as  found  by  means  of 
kites  or  pilot  balloons. 

Fourthly,  it  is  possible  to  estimate  the  upper  wind  from  the  known 
surface  wind  at  the  time.  It  is  nearly  always  found  that  for  the 
first  1,000  or  2,000  feet  above  the  surface  the  velocity  increases 


144  AIR   SERVICE   HANDBOOK. 

dii-ectly  as  the  "height  above  sea  level."'  Hence,  if  at  a  place  500 
feet  above  sea  level  the  surface  'wind  were  15  miles  per  hour,  the 
velocity  of  500  feet  above  the  "surface"  (i.  e.,  1,000  feet  above  sea 
level)  would  be  expected  to  be  30  miles  per  hour;  and  at  1,000  feet 
above  the  "surface"  (i.  e.,  1,500  feet  above  sea  level),  45  miles  per 
hour.  This  regular  increase  in  velocity  goes  on  until  the  "Gradient 
velocity"  is  reached,  after  which  the  wind  generally  remains  almost 
constant,  but  may  increase  or  decrease.  In  the  case  of  easterly 
winds  there  is  very  frequently  a  decrease  at  higher  altitudes.  The 
direction  of  the  wind  a  few  thousand  feet  up  is  slightly  changed  in  a 
clockwise  direction  from  that  of  the  surface  wind,  i.  e.,  if  the  surface 
wind  were  south  the  upper  wind  might  be  expected  to  be  south- 
southwest  or  southwest.  A  table  of  changes  in  velocity  and  direc- 
tion which  are  the  results  of  observation  is  given  in  the  Appendix. 

A  rough  rule  for  the  pilot  is  this:  The  wind  at  a  flying  height 
may  be  expected  to  be  double  that  on  the  airdrome,  and  to  have 
changed  two  points  to  the  right  of  the  direction  in  which  one  should 
leave  the  airdrome. 

It  is  a  well-known  fact  that  the  wind  is  frequently  stronger  in  the 
day  than  at  night.  This  is  nearly  always  the  case  except  in  rough 
Weather.  At  sea  the  effect  is  not  so  marked.  The  decrease  at 
night,  however,  only  takes  place  at  the  surface.  At  a  height  of 
from  1,000  to  2,000  feet  the  wind  is  stronger  at  night  than  in  the 
day.  The  cause  of  the  surface  decrease  in  velocity  at  night  seems 
to  be  the  formation  of  a  shallow  layer  of  air  on  the  ground,  which 
does  not  take  part  in  the  general  movement  of  the  air. 

"^Miile  the  velocity  of  the  wind  increases  with  height,  thegustiness 
almost  invariably  falls  off,  so  that  the  wind  is  more  steady  above 
than  at  the  surface.  No  definite  rule  can  be  given  about  the  rela- 
tion of  gustiness  and  height. 

Allied  with  gusts  are  "remous"  experienced  in  fl\ing.  These 
may  be  due  to  two  causes:  First,  a  horizontal  gust  suddenly  striking 
the  airplane  and  causing  a  temporary  change  in  its  velocity  tkrough 
the  air:  this  will  produce  a  momentary  change  in  the  lift.  Second- 
ly, there  may  be  an  ascending  or  descending  current,  which  will 
make  the  airplane  rise  or  fall. 

These  upward  or  downward  currents  may  be  caused  either  by 
trees,  buildings,  the  contour  of  the  ground,  or  they  may  be  due  to 
rising  currents  of  hot  air. 

Another  possible  occurrence  is  for  the  airplane  to  pass  into  a  mass 
of  air  mo\'ing  in  a  different  direction  to  that  in  which  it  had  been 


AIR   SERVICE   HANDBOOK. 


146 


before.     This  alt^o  causes  a  temporary  change  in  the  speed  through 
the  air,  but  this  last  cause  is  not  a  common  occurrence. 

Clouds. — The  water  vapor  in  the  air  is  chiefly  supplied  by  the 
evaporation  from  the  surface  of  oceans,  lakes,  etc.  The  air  can 
only  hold  a  certain  amount  of  water  vapor.  Hot  air  holds  more 
than  cold,  so  that  if  the  temperature  of  warm  air  which  is  saturated 
with  water  vapor  be  lowered  the  result  is  the  formation  of  fog  clouds, 
rain,  orsnow.     The  formation  of  these  is  facilitated  by  the  presence 


Ci'rofr-a'i/t  A 


NifhestAfoi/ntains  in  /'heWortd. 


'        ■     '  ^  0    ft     t» 


t 


S    ^'^ 


/Vlinbur 


-C 


Cei/ing /orZefitielin:  /^,j  '\'<^  ^    . 
I'/f /■'"  'JftfeSa//o:  r,j 


-"^ 


Fig.  59. 

of  dust  in  the  atmosphere  around  which  the  particles  of  water  can 
form. 

The  warm,  moist  aii-  near  the  sirrface  of  the  earth  rises,  meets  the 
colder  air  at  higher  levels,  which  causes  the  formation  of  clouds  out 
of  which  rain  falls. 

The  highest  clouds  are  the  cirrus,  which  occur  at  about  30,000 
feet.     They  are  composed  of  particles  of  snow  and  ice.     The  sun 
can  shine  through  them  and  they  appear  delicate,  fibrous,  and  hair- 
like.    These  clouds  are  sometimes  called  "Mares  tails." 
46643—18 10 


146  AIR  SERVICE  HANDBOOK. 

Below  at  about  20,000  feet  are  the  cirro-cumulus  clouds,  which 
consist  of  detached,  white,  globular  masses.  They  form  during 
the  hottest  months  of  the  year,  when  the  air  is  still,  and  foretell  the 
breaking  up  of  an  anticyclone. 

At  about  16,000  feet  is  a  formation  of  cloud,  sometimes  spoken  of 
as  the  "Mackerel  sky."  This  is  a  calm-weather  cloud  and  is  often 
observed  apparently  motionless  tor  some  time. 

lM0^ 


Below  this  are  the  clouds  formed  by  the  ascending  currents  of 

air  which  may  be  met  with  from  close  to  the  ground  to  kbout  10,000 

feet. 

XVI.  THEORY  OF  FLIGHT. 

Air  has  weight  inertia  and  momentxun.  It  therefore  obeys  New- 
ton's laws  and  resists  movement.  It  is  that  resistance  or  reaction 
which  makes  flight  possible. 


Fig.  61. 

Flight  is  secured  by  driving  through  the  air  a  surface  inclined 
upward  and  toward  the  direction  of  motion.  This  surface  may  be 
either  straight  or  curved. 

Chord. — The  chord  is,  for  practical  purposes,  taken  to  be  a  straight 
line  from  the  leading  edge  of  the  surface  to  its  trailing  edge. 

For  purposes  of  considering  the  lift  of  a  surface  this  line  is  drawn 
too  low.  The  neutral  lift  line,  for  a  curved  surface,  is  found  by 
means  of  wind  tunnel  research  and  it  varies  with  the  differences 
n  the  camber  of  surfaces.  This  neutral  lift  line  is  above  the ' '  chord  " 
o£  the  surface. 

In  order  to  secure  flight  the  inclination  of  the  surface  must  be 
such  that  the  neutral  lift  line  makes  an  angle  with  and  above  the 


AIR  SERVICE   HANDBOOK. 


147 


line  of  motion.  If  it  is  coincident  with  the  line  of  motion  there  is 
no  lift.  If  it  makes  an  angle  with  the  direction  of  motion  and 
below  it  then  there  is  a  pressure  tending  to  force  the  surface  down. 

Angle  of  incidence. — This  angle  is  defined  as  the  angle  the  chord 
makes  with  the  direction  of  motion.  This  is  a  bad  definition,  as  it 
leads  to  misconception  and  is  described  thus  chiefly  so  that  the 
incidence  of  a  plane  can  be  measured  easily  when  rigging  an  air- 
plane. 

The  angle  of  incidence  for  the  purposes  of  considering  flight  is 
best  described  as  the  angle  the  neutral  lift  line  makes  with  the 
direction  of  motion  relative  to  the  air.  It  will  be  no  good  giving 
a  practical  rigger  this  definition,  as  he  would  be  unable  to  find  the 
neutral  lift  line  and  he  would  probably  not  know  the  direction  of 
motion  relative  to  the  air,  whereas,  he  can  easily  put  the  machine 


nv^i 


Fig.  62. 

with  the  thrust  horizontal  and  measure  how  high  the  leading  edge 
is  above  the  trailing  edge.  This  is  explained  because  there  are 
certain  machines  which  are  described  as  ha\'ing  a  negative  angle 
of  incidence  on  the  main  plane,  and  one  might  get  the  idea  that 
these  machines  lift,  although  the  angle  is  negative.  This  is  not  so 
because  the  neutral  lift  line  must  always  be  above  the  line  of  motion. 
These  remarks  only  apply  to  cambered  surfaces.  In  the  case  of 
flat  surfaces  the  neutral  lift  line  coincides  with  the  chord.  Flat 
lifting  surfaces  are  never  used  in  a  machine. 

A  surface  acts  upon  the  air  in  the  following  manner: 

As  the  bottom  surface  meets  the  air  it  compresses  it  and  accel- 
erates it  downward.  As  a  result  of  this  definite  action  there  is,  of 
course,  an  equal  and  opposite  reaction  upward. 

The  top  surface  in  moving  forward  tends  to  leave  air  behind  it, 
thus  creating  a  semivacuum  or  rarified  area  over  the  top  of  the 
surface.  Consequently,  the  pressure  of  air  on  the  top  of  the  surface 
is  decreased,  thus  assisting  the  action  below  to  lift  the  surface 
upward. 

The  reaction  increases  approximately  as  the  square  of  the  velocity. 
Approximately  three-fifths  of  the  reaction  is  due  to  the  decrease  of 


148  AIB,   SERVICE   HANDBOOK. 

density  on  the  top  of  the  suiiace,  and  only  some  two-fifths  is  due  to 
the  upward  reaction  secured  by  the  action  of  the  1;>ottom  surface 
upon  the  air.  A  practical  point  in  respect  to  this  is  that  in  the  event 
of  the  fabric  covering  the  surface  getting  into  l)ad  c-ondition  it  is 
more  likely  to  strip  off  the  top  than  off  the  bottom. 

The  value  of  the  reaction  on  an  inclined  surface  is  given  l)y  the 

equation 

R  =  KSVH 

where  li  is  total  reaction:  A' is  a  coefficient  which  varies  with  \arious 
wing  curves:  S  is  the  surface  of  the  aerofoil:  T^is  the  velocity  of  the 
surface  through  the  air:  i  the  angle  at  which  the  aerofoil  meets  the 
stream  of  air  measm-ed  in.  Radians  and  this  angle  in  practical  flight 
is  always  very  small. 

This  formula  is  inserted  here,  as  it  is  the  fundamental  formula  of 
flight. 

The  direction  of  the  reaction  is,  at  efficient  angles  of  incidence, 
approximately  at  right  angles  to  the  neutral  line  of  the  surface,  and 
it  is,  in  considering  flight,  convenient  to  divide  it  into  two  com- 
ponent parts  or  values  thus: 

1.  The  vertical  component  of  the  reaction,  i.  e.,  lift  which  is 
opposed  to  gravity,  i.  e.,  the  weight  of  the  airplane. 

2.  The  horizontal  component,  i.  e.,  drift  (sometimes  called  re- 
sistance), to  which  is  opposed  the  thrust  of  the  propeller. 

The  direction  of  the  reaction  is,  of  coiu-se,  the  resultant  of  the  forces 
lift  and  drift.  The  lift  is  the  useful  part  of  the  reaction.  The  drift 
is  far  from  useful  and  must  be  overcome  by  the  thrust  in  order  to 
secure  the  necessary  velocity  to  produce  the  requisite  lift  for  flight. 

Drift. — The  drift  of  the  whole  airplane  (we  have  considered  only 
the  lifting  surface  heretofore)  may  be  conveniently  divided  into 
three  parts,  as  follows: 

Active  drift,  which  is  the  drift  produced  by  the  lifting  surfaces. 

Passive  drift,  which  is  the  drift  produced  by  all  the  rest  of  the 
airplane,  the  struts,  wires,  fuselage,  landing  gear,  etc.,  all  of  which 
is  known  as  the  "detrimental  siu-face." 

Skin  friction,  which  is  the  drift  produced  by  the  friction  of  the 
ail-  with  roughness  of  surface.  The  latter  is  practically  negligible, 
having  regard  to  the  smooth  surface  of  the  modern  airplane,  and  its 
com})aratiA  ely  slow  velocity  com])ared  with,  for  instance,  the  veloc- 
ity of  a  propeller  ])lade. 

IJFT-DKIFT    RATIO. 

The  importance  of  lift  to  drift  is  known  as  the  lift-drift  ratio  and 
is  of  paramount  importance,  for  it  expresses  the  "efficiency"  of  the 
airplane  (as  distinct  from  the  engine  and  propeller).  A  knowledge 
of  the  factors  governing  the  lift-drift  ratio  is,  as  will  be  seen  later. 


AIR   SERVICE   HANDBOOK. 


149 


ail  al).s(ilulc  iH'ccs.sity  lo  aiiNoiic  rcsjioriMiblf  lor  I  lie  rigt;inij  of  an 
airplane  and  the  maintenance  of  it  in  an  efficient  and  safe  condition. 

These  factors  are  as  follows: 

Velocilij. — The  greater  the  velocity  the  greater  the  proportion  of 
drift  to  lift,  and  coiise(|uently  the  less  the  efficiency.  Considering 
the  lifting  surfaces  only,  both  the  lift  and  (active)  drift,  being  com- 
ponent parts  of  the  reaction,  increase  in  the  same  pro])ortion  (as  the 
square  of  the  velocity)  and  the  efficiency  remains  the  same  at  all 
speeds.  However,  considering  the  airplane  as  a  whole,  we  must 
remember  the  passive  drift.  This  also  increases  as  the  square  of  the 
velocity,  but  there  is  no  attendant  lift.  This  passive  drift  adds 
itself  to  1  he  active  drift 
and  results  in  increas- 
ing the  proportion  of 
total  drift  to  lift. 

But  for  the  increase 
in  passive  drift  the  ef- 
ficiency of  the  airplane 
would  not  fall  with  in- 
creasing velocity,  and 
it  would  be  possible  Ity 
doubling  the  thni.st  to 
approximately  double 
the  speed  or  lift . 

This  can  never  be 
done,  but  every  effort 
is  made  to  decrease  the 
passive  drift  by  'stream  lining,"  i.  e.,  by  giving  all  "detilinentar' 
parts  of  the  airplane  a  form  by  which  they  will  pass  through  the  air 
wdth  the  least  possil)le  drift.,  The  fuselage,  struts,  wires,  etc.,  are 
all  'stream  lined"'  as  much  as  possible.  In  the  case  of  a  certain 
well-known  type  of  airplane  the  replacing  of  the  ordinary  wires  by 
'"stream-lined""  wires  added  oxcv  5  miles  an  hour  to  the  flight  speed. 

■'Head  resistance"  is  a  term  often  applied  to  passive  drift,  but  it  is 
apt  to  convey  a  wrong  impression,  as  the  drift  is  not  nearly  so  much 
the  result  of  the  head  or  forward  part  of  struts,  wires,  etc.,  as  it  is 
of  the  rarified  area  behind. 

The  aVjove  illustrates  the  flow  of  air  around  two  ol)jects  moving 
in  the  direction  of  the  arrow. 

In  the  case  of  .\  you  will  note  that  the  rarclied  area  behind  the 
object  is  very  considerable,  whereas  in  the  case  of  F^  the  air  flows 
around  it  in  such  a  way  as  to  meet  very  closely  in  the  rear  of  the 
object,  thus  decreasing  the  rarefied  area. 


Fi<;.  ti: 


160  AIR   SERVICE   HANDBOOK. 

The  greatei'  the  rarelied  area  the  less  the  density,  and  consequently 
the  less  the  pressure  of  air  upon  the  rear  of  the  object.  This  means 
that  it  will  require  more  thrust  to  overcome  this  backward  pressure. 
The  "fineness"  of  the  stream-line  shape,  i.  e.,  the  proportion  of 
length  to  width,  is  determined  by  the  velocity — the  greater  the 
velocity  the  greater  the  fineness.  The  best  degree  of  fineness  for 
any  given  velocity  is  found  by  means  of  wind-tunnel  research. 

The  practical  application  of  all  this  is,  from  a  rigging  point  of 
view,  the  importance  of  adjusting  all  stream-line  parts  to  be  dead 
on  in  the  line  of  flight. 

Angle  of  incidence. — The  most  efficient  angle  of  incidence  varies 
with  the  thrust  at  the  disposal  of  the  designer,  the  weight  to  be  car- 
ried, and  the  climb-velocity  ratio  desired. 

The  best  angles  of  incidence  for  these  varying  factors  are  found  by 
means  of  wind-tunnel  research  and  practical  trial  and  error.  Gen- 
erally speaking,  the  greater  the  velocity  the  smaller  should  be  the 
angle  of  incidence  in  order  to  preserve  a  clean  stream-line  shape 
and  prevent  the  formation  of  a  rarefied  area  and  the  formation  of 
eddies.  Should  the  angle  be  too  great  for  the  velocity  then  the  rare- 
fied area  over  the  top  of  the  surface  becomes  of  irregular  shape  with 
attendant  turbulent  eddies.  Such  eddies  possess  no  lift  value  since 
it  has  taken  power  to  produce  them;  they  represent  drift  and  ad- 
versely affect  the  lift-drift  ratio.  Also  too  great  an  angle  for  the 
velocity  will  result  in  the  under  side  of  the  surface  tending  to  com- 
press the  air  against  which  it  is  driven  rather  than  accelerate  it 
downward,  and  that  will  tend  to  produce  drift  rather  than  the  upward 
reaction  or  lift. 

From  a  rigging  point  of  view  one  must  presume  that  ca  ery  stand- 
ard airplane  has  its  lifting  surface  set  at  the  most  efficient  angle,  and 
the  practical  application  of  all  this  is  in  talcing  the  greatest  jjossible 
care  to  rig  the  surface  at  the  correct  angle  and  to  maintain  it  at  such 
an  angle.  Any  deviation  will  adversely  affect  the  lift-drift  ratio, 
i.  e.,  the  efliciency. 
f'kamber. — 

The  lifting  surfaces  are  cambered,  i.  e.,  curved,  in  order  to  decrease 
the  horizontal  component  of  the  reaction,  i.  e.,  the  drift. 

The  bottom  camber:  If  the  bottom  of  the  surface  were  flat  every 
particle  of  air  meeting  it  would  do  so  with  a  shock,  and  such  shock 
would  produce  a  very  considerable  horizontal  reaction  or  drift.  By 
curving  the  surface  such  shock  is  diminished  and  the  curve  should 
be  such  as  to  produce  a  uniform  i  not  necessarily  constant)  accelera- 
tion and  compression  of  the  air  from  the  leading  edge  to  the  trailing 
edge.  Any  unevenness  in  the  acceleration  and  compression  of  the 
air  produces  drift. 


AIR   SERVICE   HANDBOOK. 


151 


The  top  camber:  If  this  was  flat  it  would  produce  a  rarefied  area 
of  irregular  shape.  The  bad  effect  of  this  upon  the  lift-drift  ratio 
has  already  been  explained.  The  top  surface  is  then  curved  to 
produce  a  rarefied  area  the  shape  of  which  .shall  be  as  stream-line 
and  free  from  attendant  eddies  as  possible. 


Fk;.  1)4. 

The  camber  varies  with  the  angle  of  incidence,  the  a  elocity.  and 
the  thickness  of  the  surface.  Generally  speaking,  the  greater  the 
velocity  the  less  the  camber  and  angle  of  incidence.  With  infinite 
velocity  the  surface  will  be  set  at  no  angle  of  incidence  (the  neutral 
lift  line  coincident  with  the  direction  of  motion  relative  to  the  air). 


XI 


iCKtra, 


Fig.  tjo. 

and  would  be  top  and  bottom  of  pure  stream-line  form,  i.  e.,  of 
infinite  fineness.  This  is,  of  course,  canying  theorj'  to  an  absurdity, 
as  the  surface  would  then  cease  to  exist. 

The  best  cambers  for  varying  velocities,  angles  of  incidence,  and 
thicknesse.s  of  surface  are  found  bv  means  of  wind-tunnel  research. 


Id2  AIR   SERVICE   HANDBOOK. 

The  practical  application  of  all  this  is  in  taking  the  greatest  care 
to  prevent  the  surface  from  becoming  distorted  and  thus  spoiling 
the  camber  and  consequently  the  lift-drift  ratio. 

The  advantages  of  a  cambered  plane  over  a  flat  surface  are  these: 

1.  A  cambered  plane  continues  to  lift  when  the  chord  is  parallel 
to  the  line  of  liight. 

2.  The  total  lift  is  much  greater  than  that  of  a  flat  plane  of  equal 
surface. 

;'.  The  resistance  of  a  cambered  plane  in  relation  to  its  lift  is  much 
less  than  that  of  a  flat  surface. 

4.  The  top  and  bottom  cambers  can  be  made  of  different  shapes 
so  that  each  will  give  the  maximum  lift. 

5.  This  enables  suitable  spars  and  bracing  to  be  placed  inside  the 
surface  without  loss  of  efficiency. 

Aspect  ratio. — 

This  is  the  ]:)roportion  of  span  to  chord.  Thus,  if  the  span  is  for 
instance  50  feet  and  the  chord  5  feet,  the  surface  would  be  said  to 
have  an  aspect  ratio  of  10  to  1. 

If  a  flat  surface  is  acted  on  by  a  stream  of  air  at  right  angles  to  this 
surface  the  shape  does  not  very  miich  matter.  But  when  this 
surface  is  inclined  to  the  du'ection  of  motion  of  the  air  the  shape 
makes  a  great  difference. 

For  a  given  velocity  and  a  given  area  of  surface,  the  higher  tlie 
aspect  ratio  the  greater  the  reaction.  It  is  obvious,  I  think,  that 
the  gi'eater  the  span,  the  greater  the  mass  of  undisturbed  air  engaged, 
and,  as  already  explained,  the  reaction  is  partly  the  result  of  the 
mass  of  air  engaged.  The  woE-d  "undisturbed"  is  iised,  for  other- 
wise it  might  be  argued  that  whatever  the  shape  of  the  surface,  the 
same  mass  of  air  would  be  engaged.  The  word  "undisturbed" 
makes  all  the  difference,  for  it  must  be  remembered  that  the  rear 
])art  of  the  under  side  of  the  surface  engages  air  most  of  which  has 
been  deflected  downward  by  the  surface  in  front  of  it.  That  being 
so  the  rear  part  of  the  surface  has  not  the  same  opportunity  of  forcing 
the  air  downward  (since  it  is  already  fiowdng  downward)  and  secur- 
ing therefrom  an  upward  reaction  as  has  the  surface  in  front  of  it. 
It  is  therefore  of  less  value  for  its  area  than  the  front  part  of  the  sur- 
face, since  it  does  less  work  and  secures  less  reaction,  i.  e.,  lift. 
Again  the  rarefied  area  over  the  Idji  of  I  he  surface  is  most  rare  toward 
the  front  of  it,  as  owing  to  eddies  tlic  rear  of  such  area  tends  to  become 
denser. 


AIR   SERVICE   HANDBOOK.  158 

Thus  you  see  that  the  Iruiii  pari  ol  tlie  .surlacc  is  ilie  iu„.st  valuable 
troni  the  point  of  view  of  securing  an  upward  reaction  from  tlie  air- 
and  so  b>-  increasing  the  proi)ortion  of  front,  or  span,  to  chord  we 
increase  the  amount  of  reac-tion  for  a  given  velocitv  and  area  of 
surface.  That  means  a  better  proportion  of  reaction  to  weight  of 
surlace,  tiiough  the  designer  nuist  not  forget  the  drift  of  struts  and 
\nres  necessary  to  brace  up  a  surface  of  high  aspect  ratio. 

Xot  onl>  that,  but  proi-ided  the  chord  is  not  decreased  to  an  extent 
making  it  impossible  to  secure  the  best  camber  owing  to  the  thick- 
ness of  tlie  surface,  the  higher  the  aspect  ratio  the  better  the  lift-drift 
ratio.  The  reason  of  this  is  rather  obscure,  ll  is  sometimes 
advanced  that  it  is  owing  to  the  -spill  "  of  air  from  under  the  u-ing 
tips;  mth  a  high  aspect  ratio  the  chord  is  less  than  would  otherwise 
be  the  case.     Less  c-liord  results  in  smaller  wing  tips  and  conse- 


.J.I-- 


quently  less  -spill."  This,  however,  appears  to  be  a  rather  inade- 
quate reason  for  the  high  aspect  ratio  producing  the  high  lift-drift 
ratio.  Other  reasons  are  also  ad  vanced ,  but  thev  are  of  such  a  con- 
tentious nature  that  it  is  hardly  well  to  go  into*  them  here  They 
are  ot  interest  to  designers,  but  not  to  the  same  extent  to  the  prac- 
tical pilot  and  rigger.  • 
Stagger. — 

This  is  the  advancement  of  the  top  surface  relative  to  the  bottom 
surface  and  is  not  of  course  applicable  to  a  single  surface  i  e  a 
monoplane.  In  the  case  of  a  biplane  having  no  stagger,  there  will 
be  '-mterfereuce"  and  consequent  loss  of  efficiency  unless  the  -ap 
between  the  top  and  bottom  surfaces  is  equal  to  not  less  than  about 
one  and  a  half  times  the  chord.  If  less  than  that  the  air  enga-ed  by 
the  bottom  of  the  top  surface  will  have  a  tendencv  to  be  drawn  into 
the  rarefied  area  over  the  top  of  the  bottom  surface,  with  the  result 
that  the  surfaces  will  not  secure  as  good  a  reaction  as  would  other%vise 
be  the  case. 


154 


AIR   SERVICE   HANDBOOK. 


It  is  uot  practicable  to  have  a  gap  of  much  more  than  distance 
equal  to  the  chord  owing  to  the  drift  produced  by  the  great  length 
of  struts  and  wires  such  a  large  gap  would  necessitate.  By  "stag- 
gering" the  top  surface  forward,  however,  it  is  removed  from  the 
action  of  the  lower  surface  and  engages  undisturbed  air,  with  the 
result  that  the  efficiency  can  in  this  way  be  increased  by  about 
5  per  cent.  Theoretically,  the  top  plane  should  be  "staggered" 
forward  for  a  distance  equal  to  about  30  per  cent  of  the  chord,  the 
exact  distance  depending  upon  the  velocity  and  angle  of  incidence; 
but  this  is  not  always  possible  to  arrange  in  designing  an  airplane 
f — owing  to   difficulties   of    bal- 


H    K 


ance,  desired  position,  and  view 
of  pilot,  observer,  etc. 
Horizontal  equivalent. — 
The  vertical  component  of 
the  reaction,  i.  e.,  lift  varies 
as  the  horizontal  equivalent 
(H.  E.)  of  the  surface,  but  the 
drift  remains  the  same.  Then 
it  follows  that  if  the  H.  E. 
grows  less  the  ratio  of  lift  to 
drift  must  do  the  same.  The 
above  are  front  views  of  three 
surfaces  of  equal  area. 

The  top  view  has  it  full  H.  E. 
and  therefore,  from  the  point 
of  \aew  from  which  we  are  at 
the  moment  considering  efficiency,  it  has  its  best  lift-drift  ratio. 
The  two  lower  views  possess  the  same  sm*face  as  that  of  the  one  above, 
but  one  is  inclined  upward  from  its  center  and  the  other  is  straight 
but  tilted.  For  these  reasons  their  H.  E.'s  are,  as  illustrated,  less 
than  in  the  rase  of  the  first  view.  That  means  less  vertical  lift 
and  the  drift  remaining  the  same  (for  there  is  the  same  amount  of 
surface  in  each)  the  lift-drift  ratio  falls. 

The  margin  of  power. — ^This  is  the  power  a\-ailable  above  that 
necessary  to  maintain  horizontal  flight. 

The  margin  of  lift. — This  is  the  height  an  airplane  can  gain  in  a 
given  time  starting  from  a  given  altitude.  As  an  example,  thus — 
1,000  feet  the  first  minute  and  starting  fi-om  an  altitude  of  500  feet 
above  mean  sea  level. 

The  margin  of  lift  decreases  with  altitude  owing  to  the  decrease 
in  the  density  of  the  air  which  adversely  affects  the  engine.  Pro- 
vided the  engine  maintains  its  impulse  with  altitude,  then,  if  we 


AIR   SERVICE   HANDBOOK.  155 

ignore  the  problem  ol  the  jiroj^eller,  the  margin  of  lift  would  not 
disappear.  Moreover,  greater  velocity  for  a  given  power  would  be 
secured  at  a  greater  altitude,  owing  to  the  decreased  density  of  air 
to  be  overcome. 

At  the  present  time  machines  are  being  designed  to  be  most 
efficient  in  air  of  decreased  density  and  attention  is  being  paid  to 
keeping  up  the  power  of  the  engine  at  a  height  by  means  of  "blowers" 
and  "supercharges"  which  increase  the  charge  sucked  into  a  cylin- 
der at  a  high  altitude. 

The  minimum  angle  of  incidence  is  the  smallest  angle  at  which,  for  a 
given  power,  surface  (including  detrimental  surface)  and  weight, 
horizontal  flight  can  be  maintained. 

The  maa-imuvi  angle  of  incidence  is  the  greatest  angle  at  which  for 
a  given  power,  surface  (including  detrimental  surface)  and  weights 
horizontal  flight  can  be  maintained. 

The  optimum  angle  of  incidence  is  the  angle  at  which  the  lift-drift 
ratio  is  highest.  In  modern  airplanes  it  is  that  angle  of  incidence 
possessed  by  the  surface  of  the  main  ])lane  when  the  axis  of  the 
propeller  is  horizontal. 

The  best  climbing  angle  is  appro.ximately  halfway  between  the 
maximum  and  optimum  angles. 

All  present  day  aii7)lanes  are  a  compromise  between  climb  and 
horizontal  velocity. 

Exficntials  for  maximum  climb. — 

1.  Low  velocity,  in  order  to  secure  the  best  lift-drift  ratio. 

2.  Having  a  low  velocity,  a  large  surface  will  be  necessary  in 
order  to  engage  the  necessary  mass  of  air  to  secure  the  requisite 
lift. 

3.  Since  (a)  such  a  climbing  machine  will  move  along  an  upward 
sloping  path,  and  (6)  will  climb  with  its  propeller  thrust  horizontal, 
then  a  large  angle  of  the  main  plane  relative  to  the  direction  of  thrust 
will  be  necessary  in  order  to  secure  the  requisite  angle  relative  to 
the  direction  of  motion. 

4.  The  velocity  being  low  then  it  follows  that  for  that  reason 
also  the  angle  of  incidence  should  be  comparatively  large. 

5.  Since  such  an  airplane  would  be  of  low  velocity  and  therefore 
possesses  a  large  angle  of  incidence,  a  large  camber  would  be 
necessary. 

The  propeller  thrust  should  be  always  horizontal  because  the 
most  efficient  flying  machine  (having  regard  to  climb  and  velocity) 
has  so  far  been  found  to  be  an  arrangement  of  an  inclined  surface 
driven  by  a  horizontal  thrust — the  surface  lifting  the  weight  and 
the  thrust  overcoming  the  drift. 


156 


AIR   SERVICE   HANDBOOK. 


This  is  in  practice  a  far  more  efficient  arrangement  than  the 
hellicopter,  i.  e.,  the  propeller  revolving  about  a  vertical  axis  and 
producing  a  thrust  opposed  to  gravity.  If  when  climbing  the 
propeller  thrust  is  at  such  an  angle  as  to  tend  to  haul  the  airplane 
upward,  then  it  is  in  a  measure  acting  as  a  hellicopter  and  that 
means  inefficiency.  The  reason  of  a  hellicopter  being  inefficient 
in  practice  is  due  to  the  fact  that,  owing  to  mechanical  diffi- 
culties, it  is  impossible  to  construct  within  a  reasonable  weight  a 

o/1'recfion  o/  motion 


9  tf  '*'♦«  ■yo'-t^ontsi 
a  /s/~o/3&r-   fncHnjrt art  /^   //^fi 
O'r-ac n  ori  of   friofioft 


Fig.  6S. 


propeUer  of  the  requisite  dimensions.  That  being  so  it  would  be 
necessary  in  order  to  absorb  the  i>ower  of  the  engine,  to  revolve 
the  comparatively  small  surface  propeller  at  an  immensely  greater 
velocity  than  that  of  the  airplane  surface.  As  already  explained, 
the  lift-drift  ratio  falls  with  velocity  on  account  of  the  increase  in 
passive  drift.  This  applies  to  a  blade  of  a  propeller,  which  is  nothing 
but  a  revolving  surface  set  at  an  angle  of  incidence,  and  which  it  is 
impossible  to  construct  without  a  good  deal  of  detrimental  surface 
near  the  fuselage. 


'ssf)^'S/s /'Ofv  S/'rectfon.  of  ^otzon . 


JYOrizonfci/ 


Fig.  69. 


Essentiah  for  maximum  velocity. — 

The  following  are  the -essentials  for  au  airplane  of  maximum 
A'clocity  for  its  ])ower.  and  possessing  niereh-  enough  lift  to  get  ot'f 
the  ground,  but  no  margin  of  lift : 

1.  Comparatively  high  velocity. 

2.  A  comparatively  small  siu'face  because,  l)eing  of  greater  velo(it>' 
than  the  maximum  climber,  a  greater  mass  of  air  will  be  engaged 
for  a  given  surface  and  time,  and  therefore*  a  smaller  surface  will  be 
sufficient  to  secure  the  requisite  lift. 


AIR  SERVICE  HANDBOOK.  167 

'.\.  A  small  angle  relative  to  the  projjeller  thrust,  sijue  the  latter 
coincides  with  the  direction  of  motion. 

4.  A  comparativeh  small  angle  of  incidence  b>'  reason  of  the 
high  velocity. 

5.  A  comparatively  small  (•aml)er  follows  as  the  result  of  the 
small  angle  of  incidence. 

It  is  mechanicall}-  impossible  to  construct  an  airplane  of  reason- 
able weight  of  which  it  would  be  possible  to  ^'ar\■  the  above  opposing 
essentials.  Therefore,  all  airplanes  are  designed  as  a  compromise 
between  climb  and  velocity. 

As  a  rule  airplanes  are  designed  to  have  at  low  altitude  a  slight 
margin  of  lift  when  the  propeller  thrust  is  horizontal.  By  this 
means  when  the  altitude  is  reached  where  the  margin  of  lift  dis- 
appears (on  account  of  loss  of  engine  power),  and  which  is,  conse- 
quentlj-,  the  altitude  where  it  is  just  possible  to  maintain  horizontal 
flight,  the  airplane  is  flying  with  its  thrust  horizontal  and  with 
maximum  efhciency  (as  distinct  from  engine  and  propeller  effi- 
ciency). The  margin  of  lift  at  low  altitude  and  when  the  thrust  is 
horizontal  should  then  be  such  that  the  higher  altitude  at  w'hich 
the  margin  of  lift  is  lost  is  that  altitude  at  which  most  of  the  aii'- 
planes'  horizontal-flight  work  is  done.  That  insures  maximum 
velocity  when  most  required. 

Unfortunately,  when  airplanes  designed  for  fighting  are  concerned 
the  altitude  where  most  of  the  work  is  done  is  that  at  which  both 
maximum  velocity  and  maximum  margin  of  lift  for  power  are 
required.     At  present  designers  are  unable  to  effect  this. 

XVII.  STABILITY. 

Stabil'Uy  is  a  condition  whereby  an  object  disturbed  has  a  natural 
tendency  to  return  to  its  first  and  normal  position.  For  example,  a 
■weight  suspended  by  a  cord. 

Instability  is  a  (condition  whereby  an  object  disturbed  has  a  natural 
tendency  to  move  as  far  as  possible  away  from  its  first  position  with 
no  tendency  to  return.  For  example,  a  stick  balanced  vertically 
upon  the  finger. 

Natural  instability  is  a  condition  whereby  an  object  disturbed  has 
no  tendency  to  move  farther  than  it  is  displaced  by  the  force  of  the 
disturbance,  and  has  no  tendency  to  return  to  its  first  position. 

In  order  that  an  airplane  may  be  reasonably  controllable,  it  is 
necessary  for  it  to  possess  some  degree  of  stability  longitudinally, 
laterally,  and  directionally. 

Longitudinal  stability  in  an  airplane  is  its  stability  about  au  axis 
transverse  to  the  direction  of  normal  horizontal  flight,  and  without 
which  it  would  pitch  and  to.ss. 


158  AIR   SERVICE   HANDBOOK. 

Lateral  stability  is  its  stability  al)Out  its  loiigitiuliiial  axis  and 
without  which  it  would  roll  sideways. 

Directional  stability  is  its  stability  about  its  vertical  axis,  and 
without  which  it  would  have  no  tendency  to  keep  its  course. 

For  such  directional  stability  to  exist  there  must  be  "in  effect" 
more  "keel  surface"  behind  the  vertical  axis  than  there  is  in  front 
of  it.  By  "keel  surface"  is  meant  everything  which  can  be  seen 
when  looking  at  an  airplane  from  the  side — the  sides  of  the  body, 
landing  gear,  struts,  wires,  etc.  The  words  "in  effect"  are  used 
because  the  actual  area  of  the  "keel  surface"  in  front  of  the  vertical 
axis  may  be  greater  than  the  actual  surface  behind  this  axis;  but 
such  surface  will  be  much  nearer  to  the  axis  so  that  it  has  not  nearly 
so  much  leverage  as  the  surface  behind. 

The  above  illustration  represents  an  airplane  (directionally 
stable)  flying  along  a  course  B.     A  gust  striking  it  as  indicated  acts 

'  I>/rect/c>n  of  "motion  s^ae  to  >r>omofitt/m  tbrusr 
"^  /Vsrr-Ca</rr«c/ve  fce/fecf'of  ^usr 

"yer-ricle    J      ---r^ "Xj \  i  ^»ro/i/ana  or,  tti 

ruz-niny      7'  "I^A-.       \  ^fftr  t-ri^e  course  B. 


Fig.  70. 

upon  the  gieater  ]>roportion  of  "keel  surface"  behind  the  turning 
axis  and  throws  it  into  a  new  course.  The  machine,  however, 
does  not  travel  along  this  new  coiu-se,  owing  to  its  momentum  in 
the  direction  B.  It  travels  as  long  as  such  momentum  lasts  in  a 
direction  which  is  the  resultant  of  the  two  forces — thrust  and  momen- 
tum. But  the  center  line  of  the  airplane  is  pointing  in  the  direc- 
tion of  the  new  course;  therefore  its  attitude  relative  to  the  direction 
of  motion  is  more  or  less  sideways,  and  it  consequently  receives  an 
air  pressure  in  the  direction  C.  Such  pressure  acting  along  the 
"keel  siu'face"  presses  the  tail  back  toward  the  first  position,  in 
which  the  airplane  is  upon  its  course  B. 

This  tendency  to  turn  is  continually  taking  place  during  flight, 
but  in  a  well-designed  airplane  such  stabilizing  movements  are, 
for  the  most  time,  so  slight  as  to  be  imperceptible  to  the  pilot. 

If  an  airplane  was  not  stabilized  in  this  way  it  would  not  only  be 
continually  trying  to  leave  its  course,  but  it  would  also  possess  a 
dangerous  tendency  to  "nose  away"  from  the  direction  of  the  side 
gusts.  In  such  case  the  gust  shown  in  the  above  illustration  would 
turn  the  airplane  around  the  opposite  way  a  very  considerable  dis- 


AIR   SERVICE   HANDBOOK.  169 

tanr-p;  and  tlio  ri^rlit  wing  being  on  the  outside  of  llic  turn  woukl 
travel  with  greater  Avlocity  tlian  the  left  wnng.  Increased  veloeity 
means  increased  lift;  so  that,  the  riglit  ^\^^g  lifting,  the  airplane 
would  t\nn  over  sideways  very  quickly. 

Longitudhud  atnbility. — Flat  surfaces  are  longitudinally  stable, 
owing  to  the  fact  that  with  decreasing  angles  of  incidence  the  center 
line  of  pressure  (C.  P.)  moves  forward. 

The  r.  P.  is  a  line  taken  across  the  surface,  transverse  to  the 
direction  of  motion,  and  about  which  all  the  air  forces  may  be  said 
to  balance,  or  through  which  they  may  be  said  to  act. 


■^:i\ 


Fi<;.  71. 


Imagine  A  to  be  a  flat  surface,  attitude  vertical,  traveling  through 
the  air  in  the  direction  of  motion  M.  Its  C.  P.  is  then  obviously 
along  the  exact  center  line  of  the  surface  as  illustrated.  In  B,  C. 
and  D,  the  sui'faces  are  shown  with  angles  of  incidence  decreasing 
to  nothing,  and  you  will  note  that  the  C.  P.  moves  forward  with 
the  decreasing  angle.  Tlie  reason  the  C.  P.  of  an  inclined  surface 
is  forward  of  the  center  of  the  surface  is  because  the  front  of  the 
surface  does  most  of  the  work. 

Now,  should  some  gust  or  eddy  tend  to  make  the  surface  decrease 
the  angle,  i.e.,  dive,  then  the  C.  P.  moves  forward  and  pushes  the 


Fig.  72. 

front  of  the  surface  ii|).  Sliould  the  surface  tend  to  assume  too  large 
an  angle,  then  the  reverse  happens— the  C.  P.  moves  back  and  pushes 
the  rear  of  the  surface  up.  Flat  surfaces  are  then  theoretically 
stable  longitudinally.  They  are  not,  however,  used  on  account 
of  their  poor  lift-drift  ratio. 

As  already  explained,  cambered  surfaces  are  used,  and  these  are 
longitudinally  unstable  at  those  angles  of  incidence  reducing  a 
reasonable  lift-drift  ratio,  i.  e.,  at  angles  below  about  12°. 

A  is  a  cambered  surface,  attitude  approximately  vertical,  moving 
through  the  air  in  the  direction  M.  The  C.  P.  coincides  as  before 
with  the  transverse  center  line  of  the  surface.     With  decreasing 


16f)  AIR   SERVICE   HANDBOOK. 

angles  clown  to  angles  of  about  30°  the  C.  P.  moves  forward  as  in 
the  case  of  flat  surfaces;  but  the  angles  above  30°  do  no  interest  us, 
since  they  produce  a  very  low  ratio  of  lift  to  drift. 

Below  angles  of  about  30°  (see  C)  the  dipping  front  part  of  the 
surface  assumes  a  negative  angle  of  incidence  resulting  in  the  down- 
ward air  pressure  D  and  the  more  the  angle  of  incidence  is  de- 
creased, the  greater  such  negative  angles  and  its  resultant  pressure  D. 
Since  the  C.  P.  is  the  resultant  of  all  the  air  forces,  its  position  is 
naturally  affected  by  D,  which  causes  it  to  move  backward.  Now 
should  some  gust  or  eddy  tend  to  make  the  surface  decrease  its  angle 
of  incidence,  i.  e.,  dive,  then  the  C.  P.  moving  backward  and  push- 
ing up  the  rear  of  the  surface,  causes  it  to  dive  more.  Should  the 
surface  tend  to  assume  too  large  an  angle  then  the  reverse  happens; 
the  pressure  D  decreases  with  the  result  that  the  C.  P.  moves  forward 


T^i/ surf<scs  en^a^in^  sir~  i2t  /ess  ctrty/e.  _     . 

o/  inczc/ence    /■/?ar>  nyazr?  SZ//y^ctce 
G./t-/7aay/7  /'/'^ec/  ta  aero/>/srrs  <3ts^r7fe  sny/e . 
Fig.  73. 

and  pushes  up  the  front  of  the  surface  thus  increasing  the  angle 
still  farther,  the  final  result  being  a  "tail  slide." 

It  is  therefore  necessary  to  find  a  means  of  stabilizing  the  naturally 
unstable  cambered  surface.  This  is  usually  secured  by  means  of  a 
stabilizing  surface  fixed  some  distance  in  the  rear  of  the  main  sur- 
face, and  it  is  a  necessary  condition  that  the  neutral  lift  lines  of  the 
two  surfaces,  when  projected  to  meet  each  other,  make  a  dihedral 
angle.  In  other  words,  the  rear  stabilizing  surface  must  have  a 
lesser  angle  of  incidence  than  the  main  surface — certainly  not  more 
than  one-third  of  that  main  surface.  This  is  known  as  the  longi- 
tudinal dihedral. 

The  tail  plane  is  sometimes  mounted  upon  the  airplane  at  the 
same  angle  as  the  main  surface,  but  in  such  cases,  it  attacks  air  which 
has  received  a  downward  direction  from  the  main  surface,  thus: 

The  angle  at  which  the  tail  surface  attacks  the  air  (the  angle  of 
incidence)  is  therefore  less  than  the  angle  of  incidence  of  the  main 
surface. 


AIR  SERVICE  HANDBOOK.  161 

First,  imagine  the  airplane  traveling  in  the  direction  of  motion, 
which  coincides  with  the  direction  of  the  thrust  T.  The  weight  is 
of  course  balanced  about  a  0.  P.,  the  resultant  of  the  C.  P.  of  the 
main  surface  and  the  C.  P.  of  the  stabilizing  surface.  For  the  sake 
of  illustration  the  stabilizing  surface  has  been  given  an  angle  of  in- 
cidence and  therefore  has  a  lift  and  C.  P.  In  practice  the  stabilizer 
is  often  set  at  no  angle  of  incidence.    In  such  case  the  proposition 


Fl«.  74. 

remains  the  same,  but  it  is  perhaps  a  little  easier  to  illustrate  it  as 
above. 

Now,  we  will  suppose  that  a  gust  or  eddy  throws  the  machine  into 
the  lower  position.  It  no  longer  travels  in  the  direction  of  T,  since 
the  momentum  in  the  old  direction  pulls  it  off  that  course.  M  is 
now  the  resultant  of  the  thrust  and  the  momentum,  and  you  will 
note  that  this  results  in  a  decrease  in  the  angle  the  neutral  lift  line 
makes  with  the  direction  of  motion,  i.  e.,  decrease  in  the  angle  oi 
incidence  and  therefor  a  decrease  in  the  lift. 

'^aZSZHZTrrrr,         Ji^^ij-,  c^nc^^^       -^n^'e  of  ^ nccUe nee 


Momentum       ^jt^rmc^'on  of  r/jrci^n 


Yu\. 


We  will  suppose  that  this  decrease  is  2°.  Such  decrease  applies 
to  both  main  surface  and  stabilizer,  since  both  are  fixed  originally 
to  the  airplane. 

The  main  surface,  which  had  (say)  12°  angle,  has  now  only  10°, 
i.  e.,  a  loss  of  one-sixth. 

The  stabilizer,  which  had  (say)  4°  angle,  has  now  only  2°,  i.  e., 
a  loss  of  one-half. 

The  latter  has  therefore  lost  a  greater  proportion  of  its  angle  of 
incidence  and  consequently  its  lift  than  has  the  main  surface.  It 
46643—18 11 


162 


AIR   SERVICE   HANDBOOK. 


must  then  fall  relative  to  the  main  surface.  The  tail  falling,  the 
airplane  then  assumes  its  first  position  though  at  a  slightly  less 
altitude. 

Should  a  gust  throw  the  nose  of  the  airjjlane  up  then  the  reverse 
happens  and  the  airplane  will  assume  its  first  position,  though  at  a 
slightly  greater  altitude. 

Do  not  fall  into  the  widespread  error  that  the  angle  of  incidence 
varies  as  the  angle  of  the  airplane  to  the  horizontal.  It  varies 
"with"  such  angle,  but  not  "as"  anything  approaching  it.  Re- 
member that  the  stabilizing  effect  of  the  longitudinal  dihedral  lasts 
only  as  long  as  there  is  momentum  in  the  direction  of  the  first  course. 
These  stabilizing  movements  are  taking 
^   ~  ~  "  ^  place  all  the  time,  even  though  impercepti- 

^  V         ble  to  the  pilot. 

The  gyroscopic  action  of  a  rotary  engine 
will  affect  the  longitudinal  stability  when 
an  airplane  is  turned  to  the  right  or  left. 
When  a  right-hand  rotary  engine  is  fitted 
in  a  tractor  machine  the  nose  of  the  air- 
plane will  rise  when  it  is  turned  to  the  left 
and  will  fall  when  it  is  turned  to  the  right. 
In  modern  airplanes  this  tendency  is  not 
sufficiently  important  to  bother  about  ex- 
cept in  the  matter  of  spiral  descents. 

Lateral  stability  is  far  more  difficult  for 
the  designer  to  secure  than  is  longitudinal 
or  directional  stability.  Some  degree  of 
lateral  stability  may  be  secured  by  means 
of  the  lateral  dihedral,  i.  e.,  the  upward 
inclination  of  the  surface  toward  its  wing 
tips,  thus: 

Imagine  the  top  "  V  "  to  be  the  front  view 
of  a  surface  flying  away  fi'om  you.  The  horizontal  equivalent  (H .  E .) 
of  the  left  wing  is  the  same  as  that  of  the  right  wing.  Therefore, 
the  lift  of  one  wing  is  equal  to  the  lift  of  the  other,  and  the  weight 
being  situated  always  in  the  center  is  balanced. 

If  some  movement  of  the  air  causes  the  surface  to  tilt  sideways 
then  you  will  note  that  the  H.  E.  of  the  left  wing  increases  and  the 
H.  E.  of  the  right  wing  decreases.  The  left  wing,  having  the  greatest 
lift,  rises;  and  the  surface  assumes  its  first  and  normal  position. 

Unfortimately,  however,  the  righting  effect  is  not  proportional  to 
the  difference  between  the  right  and  left  H.  E.'s. 

In  the  case  of  A  the  resultant  direction  of  the  reaction  of  both 
wings  is  opposed  to  the  direction  of  gravity  or  weight.     The  two 


Fig.  76, 


AIR  SEBVICE  HANDBOOK.  163 

forces,  K.  H.  and  gravity,  are  then  evenly  balanced  and  the  surface 
is  in  a  state  of  equilibrium. 

In  the  case  of  B  you  will  note  thai  ihe  resultant  reaction  is  not 
directly  opposed  to  gra^'ity.  Tliis  results  in  the  appearance  of  M,  a 
side  pressure,  and  so  the  resultant  direction  of  motion  of  the  airplane 
is  no  longer  directly  forward,  but  is  along  a  line  the  resultant  of  the 
thrust  and  M.  In  other  words,  it  is  while  flying  forward  at  the 
same  time  mo^^ng  sideways  in  the  direction  of  M. 

In  moving  sideways  the  "keel  surface"  receives,  of  course,  a  pres- 
sure from  the  air  equal  and  opposite  to  M.  Since  such  surface  is 
greatest  in  effect  toward  the  tail  then  the  latter  must  be  pushed 
sideways.  That  causes  the  airplane  to  turn;  and  the  highest  wing 
being  on  the  outside  of  the  turn  it  has  a  greater  velocity  than  the 
lower  wing.     That  produces  greater  lift  and  tends  to  tilt  the  airplane 


3^ 

1 

-'  -  -  ^ 

'T<7    \ 

k  ^ 

1             1 
1             1 

:<,-l_^ji>i 

1            1 
.| Jr 

Fig.  77. 

still  more.  Such  tilting  tendency  is,  however,  opposed  by  the  dif- 
ference of  the  H.  E.'s  of  the  two  wings. 

It  then  follows  that  for  the  lateral  dihedral  angle  to  be  effective  such 
angle  must  be  large  enough  to  produce  when  the  airplane  tilts  a  dif- 
ference in  the  H.  E.'s  of  the  two  wings,  which  difference  must  be 
sufficient  not  only  to  oppose  the  tilting  tendency  due  to  the  airplane 
turning,  but  sufRcient  also  to  force  the  airplane  back  to  its  original 
position  of  equilibrium. 

The  above  should  make  it  clear  that  the  lateral  dihedral  is  not 
quite  so  effective  as  would  appear  at  first  sight.  Some  designers,  in- 
deed, prefer  not  to  use  it  since  its  effect  is  not  very  great  and  since  it 
must  be  paid  for  in  loss  of  H .  E.  and  consequent  loss  of  lift,  thus  de- 
creasing the  lift-drift  ratio,  i.  e.,  the  efficiency.  Also  it  is  sometimes 
advanced  that  the  lateral  dihedral  increases  the  "spill"  of  air  from 
the  wing  tips  and  that  this  adversely  affects  the  lift-drift  ratio. 

The  disposition  of  the  "keel  surface"  affects  the  lateral  stability.  It 
should  be,  in  effect,  equally  di\ided  bj^  the  longitudinal  axis  of  the 
airplane.     If  there  is  an  excess  of  "keel  surface"  above  or  below 


164  AIR   SERVICE   HANDBOOK. 

such  axis,  then  a  side  gust  striking  it  Avill  tend  to  turn  the  airplane 
over  sideways. 

The  position  of  the  center  of  gravity  affects  lateral  stability.  If  too 
low  it  produces  a  pendulum  effect  and  causes  the  airplane  to  roll 
sideways. 

If  too  high  it  acts  as  a  stick  balanced  vertically  would  act.  If 
disturbed  it  tends  to  travel  to  a  position  as  far  as  possible  from  its 
original  position.  It  would  then  tend,  when  moved,  to  turn  the 
airplane  over  sideways  and  into  an  upside-down  position. 

From  the  point  of  \dew  of  lateral  stability  the  best  position  for  the 
center  of  graxaty  is  one  a  little  below  the  center  of  drift.  This  pro- 
duces a  little  lateral  stability  without  any  marked  pendulum  effect. 

Propeller  torque  affects  lateral  stability.  An  airplane  tends  to 
turn  over  sideways  in  the  opposite  direction  to  that  in  which  the 
propeller  revolves. 

This  tendency  is  offset  by  increasing  the  angle  of  incidence  (and 
consequently  the  lift)  of  the  side  tending  to  fall;  and  it  is  always 

J^eutral  arr^/e 
0/  Incidence 


advisable,  if  practical  considerations  allow  it,  to  also  decrease  the 
angle  upon  the  other  side  an  equal  amount.     In  that  way  it  is  not 
necessary  to  depart  so  far  from  the  normal  angle  of  incidence  at 
which  the  lift-drift  ratio  is  highest. 

Wash  in  is  the  term  applied  to  the  increased  angle. 

Wash  out  is  the  term  applied  to  the  decreased  angle. 

Both  lateral  and  directional  stability  may  be  improved  by  washing 
out  the  angle  of  incidence  on  both  sides  of  the  surface. 

The  decreased  angle  decreases  the  drift  and  therefore  the  effect 
of  gusts  upon  the  wing  tips,  which  is  just  where  they  have  the  most 
effect  upon  the  airplane,  owing  to  the  distance  from  the  turning  axis. 

The  wash  out  also  renders  the  ailerons  more  effective,  as,  in  order 
to  operate  them  it  is  not  then  necessary  to  give  them  such  a  large 
angle  of  incidence  as  would  otherwise  be  required. 

The  less  the  angle  of  incidence  of  the  ailerons  the  better  their  lift- 
drift  ratio,  i.  e.,  their  efficiency.  For  the  same  amount  of  move- 
ment therefore  the  ailerons  are  more  efficient  when  attached  to  the 
surface  with  washed  out  angle  of  incidence. 


AIR  SERVICE   HANDBOOK.  165 

The  advantafi;eti  of  wash  in  inu.st  oi'  course  l^e  paid  lor  with  some  loss 
of  lift,  as  the  lift  decreases  with  the  decreased  anglp. 

Banking. — An  airplane  turned  off  its  course  to  right  or  left  does 
not  at  once  proceed  along  its  new  course.  Its  momentum  in  the 
direction  of  its  first  course  causes  it  to  travel  along  a  line  the  resultant 
of  such  momentum  and  the  thrust.  In  other  words,  it  more  or  less 
skids  sideways  and  away  from  the  center  of  the  turn.  Its  lifting 
surfaces  do  not  then  meet  the  air  in  their  correct  attitude,  and  the 
lift  may  fall  to  such  an  extent  as  to  become  less  than  the  weight,  in 
which  case  the  airplane  must  fall.  This  bad  effect  i.s  minimized  by 
"banking,"  i.  e.,  tilting  the  airplane  sideways.  The  bottom  of  the 
lifting  surface  is  in  that  way  opposed  to  the  aii-  through  which  it  is 
moving  in  the  direction  of  the  momentum  and  receives  an  opposite 
ail-  pressure.  The  rarified  air  over  the  top  of  the  surface  is  rendered 
still  more  rare  and  thLs  of  course  a.ssists  the  air  pressure  in  opposing 
the  momentum. 

The  velocity  of  the  "skid"  or  sideways  movement  is  then  only 
such  as  is  necessary  to  secure  an  air  pressure  equal  and  opposite  to 
the  centrifugal  force  of  the  turn.  The  sharper  the  turn,  the  greater 
the  effect  of  the  centrifugal  force,  and  therefore  the  steeper  should 
be  the  ''bank." 

The  position  of  the  center  of  gravity  effects  banking.  A  low 
C.  G.  will  tend  to  swing  outward  from  the  center  of  the  turn,  and  will 
cause  the  airplane  to  bank — perhaps  too  much,  in  which  case  the 
pilot  must  remedy  matters  by  operating  the  ailerons. 

A  high  C.  G.  also  tends  to  swing  outward  from  the  center  of  the 
turn.  It  will  tend  to  make  the  airplane  bank  the  wrong  wa}-,  and 
such  effect  must  be  remedied  by  means  of  the  ailerons. 

The  pleasantest  machine  from  a  banking  point  of  view  is  one  in 
which  the  0.  G.  is  a  little  below  the  center  of  drift.  It  tends  to  bank 
the  airplane  the  right  way  for  the  turn  and  the  pilot  can,  if  neces- 
sary, perfect  the  bank  by  means  of  the  ailerons. 

The  disposition  of  the  "keel  surface"  affects  banking.  It  should 
be,  in  effect,  evenly  divided  by  the  longitudinal  axis.  An  excess 
of  "keel  surface"  above  the  longitudinal  surface  will  when  banking 
receive  an  air  pressure,  causing  the  airplane  to  bank  perhaps  too 
much.  An  excess  of  "keel  surface"  below  the  axis  has  the  reverse 
effect. 

Side  Hiipping. — This  usually  occurs  as  a  result  of  overbanking.  It 
is  always  the  result  of  the  airplane  tilting  sideways  and  thus  decreas- 
ing the  horizontal  equivalent  and  therefore  the  lift  of  the  surface. 
An  excessive  bank  or  sideways  tilt  results  in  the  H.  E.  and  there- 
fore the  lift  becoming  less  than  the  weight,  when  of  c()urs(\  the  air- 
plane must  fall,  i.  e.,  side  slip. 


166 


AIR  SERVICE   HANDBOOK. 


When  making  a  very  sharp  turn  it  is  necessary  to  hsuik  very 
steeply  indeed.  If  at  the  same  time  the  longitudinal  axis  of  the 
airplane  remains  approximately  horizontal  then  there  must  be  a  fall 
and  the  direction  of  motion  will  be  the  resultant  of  the  thrust  and 
the  fall,  as  illustrated  in  sketch  A.  The  lifting  siu'faces  and  the 
controlling  surfaces  are  not  then  meeting  the  air  in  the  correct  atti- 
tude, with  the  result  that  in  addition  to  falling  the  airplane  will 
probably  become  quite  unmanageable. 

The  pilot,  however,  prevents  such  a  state  of  affairs  from  happening 
by  "nosing  down,"  i.  e.,  by  operating  the  rudder  to  turn  the  nose 
of  the  airplane  downward  and   toward   the  direction  of  motion  a 


F IG.  79. 

illustrated  in  sketch  B.  This  results  in  the  higher  wing  which  is 
on  the  outside  of  the  turn  traveling  with  greater  velocity,  and  there- 
fore securing  a  greater  reaction  than  the  lower  wing,  thus  tending 
to  tilt  the  airplane  over  still  more.  The  airplane  may  be  now 
almost  upside  down,  but.  its  attitude  relative  to  the  direction  of 
motion  is  correct  and  the  controlling  surfaces  are  all  of  them  working 
efficiently.  The  recovery  of  a  normal  attitude  relative  to  the  earth 
is  then  made  as  illustrated  in  sketch  C  by  gently  pulling  back  the 
elevator  control. 

The  pilot  must  then  learn  to  know  just  the  angle  of  bank  at  which 
the  margin  of  lift  is  lost,  and,  it  a  sharp  t,urn  necessitates  banking 
beyond  that  angle,  he  must ' '  nose  down . "     In  this  matter  of  banking 


AIR   SERVICE   HANDBOOK.  167 

and  '"nosing  down"  and  indeed  regarding  stalnlity  and  control  gen- 
erally, the  golden  rule  for  all  but  verj-  experienced  pilots  should  be: 

"Keep  the  airplane  in  such  an  attitude  that  the  air  pressure  is 
always  directly  in  the  pilot's  face." 

The  airjilane  is  then  always  engaging  the  air  as  designed  to  do  so, 
and  both  lifting  and  controlling  surfaces  are  acting  efficiently. 

Spinning. — A  spin  is  due  primarily  to  the  loss  of  flying  speed. 
It  is  quite  different  from  a  quick  turn  of  small  radius.  The  usual 
form  of  spin  is  for  the  machine  to  come  down  at  an  angle  of  aljout  00° 
^vith  the  tail  turning  rapidly,  and  the  angle  may  become  steeper  and 
steeper.  When  this  happens  the  attitude  of  the  lifting  .surfaces  to 
the  direction  of  motion  is  roo  great,  and  there  is  a  greater  pressure 
trying  to  collapse  the  wings  than  there  ought  to  l)e. 

0^^dng  to  the  small  radius  of  such  a  spiral  the  mass  of  the  airplane 
may  gain  a  rotarj^  momentum  greater  in  effect  than  the  air  pressure 
of  the  "'keel  surface"  or  controlling  surfaces  opposed  to  it:  when 
once  such  a  condition  occurs  it  is  difhcult  to  see  what  can  be  done 
by  the  pilot  to  remedy  it. 

In  this  connection  e^ery  pil(jt  of  an  airplane  fitted  with  a  rotary 
engine  should  bear  in  mind  the  gyroscopic  effect  of  such  engines. 
In  the  ca.se  of  such  an  engine  fitted  to  a  tractor  machine  its  effect  is 
to  depress  the  nose  if  a  right-hand  turn  i.s  made.  The  sharper  the 
turn  the  greater  such  effect. 

An  effect  which  may  render  the  airplane  uiunanageal)le  if  the 
spiral  is  one  of  vexy  small  radius  and  the  engine  is  revohing  with 
sufficient  speed  to  produce  a  material  gjn'oscopic  effect.  .Such 
gjroscopic  effect  will,  however,  assist  the  pilot  to  navigate  a  small 
spiial  if  he  turns  his  machijie  the  opposite  way.  The  a.ssistance  will 
only  be  slight  Ijecau.so  (he  engine  should  of  course  be  throttled  down 
for  a  spiral  descent. 

Nearly  all  machines  can  l)e  made  to  sjiin  more  or  le.ss.  Some  are 
harder  than  others. 

In  order  to  get  into  a  spin  pull  back  the  control  until  the  machine 
is  almost  stalled,  then  kick  the  rudder  one  way  or  the  other  and  the 
machine  will  spin. 

To  get  out  of  a  spin  tluoltle  back  the  engine,  put  all  controls  in 
neutral  and  then  slightly  i)ush  forward  the  elevator  control. 

All  controls  must  be  put  in  neutral  to  give  the  machine  a  chance 
of  regaining  flying  speed.  Any  control  which  is  in  action  increases 
the  drift  of  the  aii'plane  and  prevents  this. 

As  machines  are  not  specially  designed  to  take  the  stresses  of  a 
spin,  machines  should  not  be  spun  without  the  sanction  of  the  de- 
signers and  testers. 

Gliding  descent  uithout  propeller  thrust. — All  airplanes  are  or  should 
be  designed  to  assume  their  correct  gliding  angle  when  the  power 


168 


AIR  SERVICE   HANDBOOK. 


and  thrust  is  cut  off.  This  relieves  the  ])ilut  of  work,  worry,  and 
danger  -should  he  find  himself  in  a  fog  or  cloud.  The  pilot,  although 
he  may  not  realize  it,  maintains  the  correct  attitude  of  the  airjjlane 
by  observing  its  position  relative  to  the  horizon.  Flying  into  a  fog 
or  cloud,  the  horizon  is  lost  to  view,  and  he  must  rely  upon  his 
instruments:  (1)  The  compass  for  direction:  (2)  an  inclinometer, 
mounted  transA^ersely  to  the  longitudinal  axis,  for  lateral  staliility; 
and  (3)  an  inclinometer  mounted  parallel  to  the  longitudinal  axis, 
or  the  air  speed  indicator  which  will  indicate  a  nose-down  position 
by  increase  in  air  speed,  and  a  tail-down  position  by  a  decrease. 

The  pilot  is  then  under  the  necessity  of  watching  three  instru- 
ments and  manipulating  his  three  controls  to  keep  the  instrument 
indicating  longitudinal,  lateral,  and  dii'ectional  stability.  That  is 
a  feat  beyond  the  capacity  of  the  crdinarj-  man.  If,  however,  by 
the  simple  movement  of  throttling  down  the  power  and  the  thrust 


Fig.  80. 

he  can  be  relieved  of  looking  after  thclungitiidinal  stability  he  then 
has  only  two  instruments  to  watch . 

Airplanes  are  therfore  designed,  or  should  be,  so  that  the  center 
of  gravity  is  slightly  forward  of  the  center  of  lift.  The  airplane  is 
then,  as  a  glider,  nose  heavy,  and  the  distance  the  C.  G.  is  placed 
in  advance  of  the  C.  L.  should  be  such  as  to  insure  a  gliding  angle 
producing  a  velocity  the  same  as  the  normal  fljdng  speed. 

In  order  that  this  nose-heavy  tendency  should  not  exist  when  the 
thrust  is  working  and  descent  not  required  the  center  of  thrust  is 
placed  a  little  below  the  center  of  drift  and  resistance,  and  thus 
tends  to  pull  up  the  nose  of  the  airplane. 

The  distance  the  center  of  thrust  is  placed  below  the  center  of 
drift  should  be  such  as  to  produce  a  force  equal  and  ojjposite  to  that 
due  to  the  C.  G.  being  forward  of  the  ('.  L. 


AIR   SERVICE   HANDBOOK.  169 

Looping  and  upside-down  Jlying. — If  a  loop  is  desired  it  is  best  to 
throttle  down  the  engine  at  a  point  A  when  the  top  of  the  loop  is 
reached.  The  C.  G.  being  forward  of  the  C.  P.  causes  the  airplane 
to  nose  down  and  assists  the  jiilot  in  making  a  reasonably  small  loop 
along  the  course  C  and  in  securing  a  quick  recovery.  If  the  engine 
is  not  thi'ottled  down  then  the  airplane  may  be  expected  to  follow 
the  course  D  which  results  in  a  longer  nose  dive  than  in  the  case  of 
the  course  C. 

When  pulling  the  machine  out  of  the  nose  dive  a  steady  and  gentle 
movement  of  the  elevator  is  necessary.  A  jerky  movement  may 
change  the  direction  of  motion  so  suddenly  as  to  produce  dangerous 


-y^- 


""^-^ 


Fig.  si. 


air  stresses  upon  tlu"  surfaces,  in  which  case  there  is  a  possibility  to 
collapse. 

If  an  upside-down  flight  is  desired,  the  engine  may  or  may  not  be 
throttled  down  at  point  A.  If  not  throttled  down,  then  the  elevator 
must  bo  operated  to  secure  a  com-se  approximately  in  the  direction 
P>.  If  it  is  throttled  down,  then  the  course  must  be  one  of  a  steeper 
angle  than  15  or  there  will  be  danger  of  stalling. 

To  start  a  loop  it  is  necessary  with  some  machines  to  push  the  nose 
down  in  order  to  gather  speed.  Some  machines  will  go  straight  over 
from  the  horizontal  flying  position.  In  any  case  the  elevator  should 
be  pulled  back  gradually  until  the  machine  has  got  very  nearly  to 
the  position  A,  when  it  should  be  pulled  back  as  far  as  it  will  go  so 


170  AIR   SERVICE   HANDBOOK. 

as  to  bring  the  machine  over  top  of  the  loop.  As  the  machine  goes 
over,  the  rudder  must  be  put  over;  that  is,  in  a  machine  which 
flies  with  left  rudder,  the  rudder  must  be  put  over  to  the  right  as  the 
machine  goes  over  the  top,  otherwise  the  loop  will  not  be  clean. 
If  the  elevator  control  is  pulled  too  roughly  the  machine  will  stall 
before  it  goes  over  the  top  and  will  not  complete  the  loop. 

In  machines  which  will  be  used  for  looping  and  nose  diving,  and 
also  in  high-powered  weight-carrying  machines,  the  greatest  care 
must  be  taken  about  the  tension  of  the  incidence  and  flying  wires. 
Remember  that  a  machine  is  designed  to  take  certain  stresses  when 
flying.  If  the  bracing  wires  are  tightened  to  such  an  extent  that 
they  "sing"  when  vibrated  it  means  that  the  spars,  etc.,  have  a 
considerable  initial  strain  for  which  the  machine  is  not  designed- 
All  bracing  wires  should  therefore  never  be  tight  but  should  only 
have  the  slackness  taken  out  of  them  and  nothing  more. 

XVIII.  NOMENCLATURE. 

Aerofoil. 

A  thin  wing-like  structure,  flat  or  curved,  designed  to  obtain 
reaction  upon  its  surfaces  from  the  air  through  which  it  moves. 
Aileron. 

A  movable  auxiliary  surface,   used  for  the  control  of  rolling 
motion,  i.  e..  rotation  about  the  fore  and  aft  axis. 
Aircraft. 

Any  form  of  craft  designed  for  the  navigation  of  the  air. 
Airplane. 

A  form  of  aircraft  heavier  than  air,  which  has  wing  suifaces  for 
sustentation.  with  stabilizing  surfaces,  rudders  for  steering, 
and  power  plant  for  propulsion  through  the  air.  The  landing 
gear  may  be  suited  for  either  land  or  water  use. 

Pusher,  a  type  of  airplane  with  the  propeller  or  propellers 

in  the  rear  of  the  wings. 
Tractor,  a  type  of  airplane  with  the  propeller  or  propellers 
in  front  of  the  -wings. 
Air-speed  meter. 

An  instrument  designed  to  measure  the  velocity  of  an  aircraft 
with  reference  to  the  air  through  which  it  is  moving. 
Altimeter. 

An  instrument  mounted  on  an  aircraft  to  continuously  indicate 
its  height  above  the  surface  of  the  earth. 
Anemometer. 

An  instrument  for  measuring  the  velocity  of  the  wind  or  air 
currents  with  reference  to  the  earth  or  some  fixed  body. 


AIR   SERVICE   HANDBOOK.  171 

Angle. 

Of  attack,  the  antrle  between  the  direction  of  the  relative  \vind 

and  the  chord  of  an  aerofoil,  or  the  fore  and  aft  axis  of  a  body. 

Critical,  the  angle  of  attack  at  which  the  lift  is  maximum. 

Gliding,  the  angle  the  flight  path  makes  ^vith  the  horizontal 

when  flying  in  still  air  under  the  influence  of  gravity  alone. 

Aspect  ratio. 

The  ratio  of  spread  to  chord  of  an  aerofoil. 
Axes  of  an  aircraft. 

Three  fixed  lines  of  referejice;  usually  centroidal  and  mutually 
rectangular. 

Longitudinal  axis,  usually  parallel  to  the  axis  of  the  pro- 
peller, is  the  principal  longitudinal  axis  in  the  plane  of 
symmetry.     Sometimes  called  ''fore  and  aft  axis." 
Vertical  axis,  the  axis  perpendicular  to  the  above  in  the 

plane  of  symmetry. 
Trans\erse  or  lateral  axis  is  the  third  axis  perpendicular  to 

the  other  two.     Sometimes  called  "athwartship  axis." 
In  mathematical  discussions  the  tirst  of  these  axes  is  called 
the  X  axis,  the  second  Z  axis,  and  the  third  the  Y  axis. 
Ballonet. 

A  small  balloon  within  the  interior  of  a  balloon  or  dirigible  for 
the  purpose  of  controlling  the  ascent  or  descent,  and  for  main- 
taining pressure  on  the  outer  envelope  to  prevent  deforma- 
tion. The  ballonet  is  kept  inflated  with  air  at  the  required 
pressure,  under  the  control  of  a  blower  and  valves. 
Balloon. 

A  form  of  aircraft  comprising  a  gas  bag  and  a  oar.  whose  susteu- 
tation  depends  on  the  buoyance  of  the  contained  gas.  which 
is  lighter  than  air. 

Captive,  a  balloon  restrained  from  free  flight  by  means  of  a 

cable  attaching  it  to  the  earth. 
Kite,  an  elongated  form  of  captive  balloon  fitted  with  tail 
appendages  to  keep  it  headed  into  the  wind  and  deriving 
increased  lift  due  to  its  axis  being  inclined  to  the  wind. 
Bank. 

To  incline  an  airplane  laterally,  i.  e..  to  rotate  it  about  the  fore 
and  aft  axis.     Right  bank  is  to  incline  the  airplane  with  the 
right  wing  down. 
Barograph . 

An  instrument  used  to  record  variations  in  barometric  pressure. 
In  aeronautics  the  charts  on  which  the  records  are  made  are 
prepared  to  indicate  altitudes  directly  instead  of  barometric 
pressure. 


172  AIR   SERVICE   HANDBOOK. 

Biplane. 

A  form  of  airplane  in  which  the  main  supporting  surface  is 
divided  into  parts,  one  above  the  other. 
Body  of  an  airplane. 

A  structure  usually  inclosed,  which  contains  in  a  stream-line 
housing  the  power  plant,  fuel,  passengers,  etc. 
Oabre. 

A  flying  attitude  in  which  the  angle  of  attack  is  greater  than 
normal;  tail  down;  down  by  the  stern — tail  low. 
Camber. 

The  convexity  or  rise  of  a  curve  of  an  aerofoil  from  its  chord, 
usually  expressed  as  the  ratio  of  the  maximum  departure  of 
the  curve  from  the  chord  as  a  fraction  thereof.  Top  camber 
refers  to  the  top  siu^ace  of  an  aerofoil,  and  bottom  camber  to 
the  bottom  surface. 
Capacity. 

Lifting,  the  maximimi  load  of  an  aircraft. 

Carrying,  excess  of  the  lifting  capacity  over  the  dead  load  of  an 
aircraft,  which  latter  includes  structure,  power  plant,  and 
essential  accessories. 
Center. 

The  point  in  which  a  set  of  effects  is  assumed  to  l)e  accumulated 
producing  the  same  effect  as  if  all  were  concentrated  at  this 
point. 

Of  buoyancy,  the  center  of  gravity  of  the  fluid  displaced  by 

the  floating  body. 
Of  pressure  of  an  aerofoil,  the  point  on  the  chord  of  an  ele- 
ment of  an  aerofoil,  prolonged  if  necessary,  through  which 
at  any  instant  the  line  of  action  of  the  resultant  air  force 
passes. 
Of  pressure  of  a  body,  the  point  on  the  axis  of  a  bgdy,  pro- 
longed if  necessary,  through  which  at  any  instant  the  line 
of  action  of  the  resultant  air  force  passes. 
Chord . 

Of  an  aerofoil  section,  a  right  line  tangent  to  the  under  curve 

of  the  aerofoil  section  at  the  Front  and  rear. 
Length,  the  length  of  the  chord  is  the  length  of  an  aerofoil  sec- 
tion projected  on  tlio  chord,  extended  if  Tiecessary. 
Controls . 

A  general  term  ap])lying  to  tiie  means  provided  for  operating  the 
devices  used  to  control  speed,  direction  of  flight,  and  attitude 
of  an  aircraft. 
Dirigible. 

A  form  of  balloon  the  outer  envelope  of  which  is  of  elongated 
form,  provided  with  a  ])ropelliiig  system,  car,  rudders,  and 
stabilizing  surfaces. 


AIR  SERVICE   HANDBOOK.  178 

Dope. 

A  general  term  applied  to  the  materials  used  ia  treating  the 
cloth  surface  of  airplane  members  to  increased  strength,  pro- 
duce tautness,  and  act  as  a  filler  to  maintain  air  tightness; 
usually  of  the  cellulose  type. 
Drag. 

The  total  resistance  of  motion  through  the  air  of  an  aircraft,  i.  e., 
the  sum  of  the  drift  and  head  resistance. 
Drift. 

The  component  of  the  resultant  wind  pressiu'e  on  an  aerofoil  or 
wing  surface  parallel  to  the  air  stream  attacking  the  surface. 
Elevator. 

A  hinged  surface  for  controlling  the  longitudinal  attitude  of  an 
aircraft,  i.  e.,  its  rotation  about  the  athwartship  axis. 
Engine,  right  or  left  hand. 

The  distinction  between  a  right-hand  and  a  left-hand  engine 
depends  on  the  rotation  of  the  output  shaft,  whether  this  shaft 
rotates  in  the  same  direction  as  the  crank  or  not.  A  right- 
hand  engine  is  one  in  which  when  viewed  from  the  output 
shaft  end,  the  shaft  is  seen  to  rotate  anticlockwdse. 
Entering  edge. 

The  foremost  part  of  an  aerofoil. 
Fins. 

Small  planes  on  the  aircraft  to  promote  stability;  for  example 
vertical  tail  fins,  etc. 
Flight  path; 

The  path  of  the  center  of  gravity  of  an  aircraft  with  reference  to 
the  air. 
Float: 

That  portion  of  the  landing  gear  of  an  aircraft  which  provides 
buoyancy  when  it  is  resting  on  the  siu'face  of  the  water. 
Fuselage.     (See  body.) 
Gap: 

The  distance  between  the  projections  on  the  vertical  axis  of  the 
entering  edges  of  an  upper  and  lower  wing  of  a  biplane. 
Glide: 

To  fly  without  power. 
Head  resistance: 

The  total  resistance  to  motion  through  the  air  of  all  parts  of 
an  aircraft  not  a  part  of  the  main  lifting  surface.     Sometimes 
termed  '  *  parasite  resistance . ' ' 
Helicopter: 

A  form  of  aircraft  whose  support  in  air  is  derived  from  the  ver- 
tical thrust  of  large  propellers. 


174  AIR  SERVICE  HANDBOOK. 

Inclinometei' : 

An  instrument  for  measuring  the  angle  made  by  any  axis  of  an 
aircraft  with  the  horizontal. 
Keelplane  area: 

The  total  effective  area  of  an  aircraft  which  acts  to  prevent 
skidding  or  side  slipping. 
Landing  gear: 

The  understructure  of  an  aircraft  designed  to  carry  the  load  when 
resting  on,  or  running  on,  the  surface  of  the  land  or  water. 
Leeding  edge.     (See  entering  edge.) 
Leeway : 

The  angular  deviation  from  a  coursse  over  the  earth,  due  to  cross 
currents  of  wind. 
Lift: 

The  component  of  the  force  due  to  the  air  pressvue  of  an  aerofoil, 
resolved  perpendicular  to  the  flight  part  in  a  vertical  plane. 
Longeron : 

A  fore-and-aft  member  of  the  framing  of  an  airplane  body,  or  of 
the  floats,  usually  continuous  across  a  number  of  points  of 
support. 
Metacenter : 

The  point  of  intersection  of  a  vertical  line  through  the  center 
of  gravity  of  the  fluid  displaced  by  a  floating  body  when  it  is 
tipped  through  a  small  angle  from  its  position  of  equilibrium 
and  the  inclined  line  which  was  vertical  through  the  center 
of  gra\'ity  of  the  body  when  in  equilibrium.  There  is  in 
general  a  different  metacenter  for  each  type  of  displacement 
of  the  floating  body. 
Monoplane: 

A  form  of  airplane  whose  main  supportiilg  surface  is  disposed 
as  a  single  wing  on  each  side  of  the  body. 
Nacelle.     (See  body.) 
Nose  dive: 

A  dangerously  steep  descent,  head  on. 
Ornithopter: 

A  form  of  aircraft  deriving  its  .support  and  propelling  force  from 
flapping  wings. 
Pilot  tube: 

A  tube  with  an  end  open  square  to  the  fluid  stream,  used  as  a 
detector  of  an  impact  pressure.  More  usually  associated  with 
a  concentric  tube  surrounding  it,  having  perforations  normal 
to  the  axis  for  indicating  static  pressure.  The  velocity  of 
the  fluid  can  be  determined  from  the  difference  between  the 
impact  pressure  and  the  static  pressure.  This  instrument  is 
often  used  to  determine  the  velocity  of  an  aircraft  through 
the  air. 


AIR   SERVICE   HANDBOOK.  176 

I'ropeller: 

Disk  area  of,  tlie  total  area  oi'  the  <lisk  swopt  hy  tho  pmpoller 

tips. 
Right-hand,  one  in  which  the  helix  is  right-handed. 
Race  of,  the  air  stream  delivered  by  the  propeller. 
Slip  of.     This  term  applies  to  propeller  action  and  is  the  differ- 
ence between  the  actual  velocity  of  advance  of  an  aircraft 
and  the  speed  calculated  from  the  known  pitch  of  the  pro- 
peller and  its  number  of  revolutions. 
I'ylon; 

A  mai'ker  of  a  course. 
Rudder. 

A  hinged  or  pivoted  surface,  usually  more  or  less  Hat  or  stream- 
lined, used  for  the  purpose  of  controlling  the  attitude  of  an 
aircraft  about  its  vertical  axis  when  in  motion. 
Side'slipping. 

Sliding  toward  the  inside  of  a  turn.     It  is  due  to  excessive 
amount  of  bank  for  the  turn  being  made,  and  is  the  opposite 
of  skidding. 
Skidding. 

Skidding  sideways  in  flight  away  from  the  center  of  the  turn . 
It  is  usually  caused  by  insufficient  banking  in  a  turn,  and 
is  the  opposite  of  side  slipping. 
Spread. 

The  maximum    distance  laterally  from  tip  to    tip  of   an  air- 
plane's wings. 
Stability. 

The  quality  of  an  aircraft  in  flight  which  causes  it  to  return  to  a 
condition  of  equilibrium  when  meeting  a  disturbance.     (This 
is  sometimes  called  "Dynamical  stability.  ") 
Directional,  stability  with  reference  to  the  vertical  axis. 
Inherent,  stability  of  an  aircraft  due  to  the  disposition  and 

arrangement  of  its  fixed  parts. 
Lateral,  stability  with  reference  to  the  longitudinal,  or  fore 

and  aft  axis. 
Longitudinal,  stability  with  reference  to  the  lateral,   or 
athwartship  axis. 
Stabilizer.     (See  Fins.) 

This  term  usually  applies  to  the  fixed  horizontal,  tail  surface  of 

an  airplane. 
Mechanical,  any  automatic  device  designed  to  secure  stability 
in  flight.  ' 


176  AIR  SERVICE  HANDBOOK. 

Stagger. 

The  amount  of  advance  of  the  entering  edge  of   the    upper 
wing  of  a  biplane  over  that  of  the  lower;  it  is  considered 
positive  when  the  upper  surface  is  forward. 
Stalling. 

A  term  describing  the  condition  of  an  airplane  wliich  from  any 
cause  has  lost  the  relative  speed  necessary  for  steerage  way 
and  control. 
Statoscope. 

An  instrument  to  detect  the  existence  of  a  small  rate  of  ascent 
or  descent,  principally  used  in  ballooning. 
Stream-line  flow. 

A  term  in  hydi'omechanics  to  describe  the  condition  of  contin- 
uous flow  of  a  fluid,  as  distinguished  from  eddying  flow  where 
discontinuity  takes  place. 
Stream-line  shape. 

A  shape  intended  to  avoid   eddying  or  discontinuity  and  to 
preserve  stream-line  flow,  thus  keeping  resistance  to  progress 
at  a  minimum. 
Strut. 

A   compression   member  of  a  truss  frame;   for  instance,    the 
vertical  members  of  the  wing  truss  of  a  biplane. 
Sweep  back. 

The  horizontal  angle  between  the  lateral  axis  of  an  airplane 
and  the  entering  edge  of  the  main  planes. 
Tail. 

The  rear  portion  of  the  aircraft  to  which  are  usually  attached 
rudders,  elevators,  and  fins. 
Trailing  edge. 

The  rearmost  portion  of  an  aerofoil. 
Triplane. 

A  form  of  airplane  whose  main  supporting  surfaces  are  divided 
into  three  parts  superimposed. 
Wake  gain. 

Due  to  the  influence  of  skin  friction,  eddying,  etc.,  a  vessel  in 
moving  forward  produces  a  certain  forward  movement  of  the 
fluid  surrounding  it.  The  effect  of  tliis  is  to  reduce  the 
effective  resistance  of  the  hull,  and  this  effect,  due  to  the 
forward  movement  of  the  wake,  is  termed  the  "wake  gain." 
In  addition  to  tliis  effect  the  forward  movement  of  this  body 
of  fluid  reduces  the  actual  advance  of  the  propeller  through 
the  surrounding  medium,  thereby  reducing  the  propeller 
horsepower. 


AIR  SERVICE  HANDBOOK.  177 

Warp. 

To  change  the  form  of  the  mng  by  twisting  it,  usHally  by  chang- 
ing the  inclination  of  the  rear  spar  relative  to  the  front  epar. 
Wings. 

The  main  supporting  surfaces  of  an  airplane. 
Wing. 

Loading,  the  weight  carried  per  unit  area  of  supporting  surface. 
Rib,  a  fore  and  aft  member  of  the  wing  structure,  used  to  support 

the  covering  and  to  give  the  wing  section  its  form. 
Spar,  an  athwartship  member  of  the  wing  structure  resisting 
tension  and  compression. 
Yaw. 

To  swing  off  the  course  about  the  vertical  axis,  owing  to  gusts 
or  lack  of  directional  stability. 
Angle  of,  the  temporary  angiilar  deviation  of  the  fore  and 
aft  axis  from  the  course. 
46643—18 12 


APPENDIX. 


Table  1. — Shomng  fall  of  barometer  with  height. 

The  height  of  barometer  was  taken  as  30  inches  at  mean  sea  level 
with  temperature  32°  F.  This  approximates  to  a  sounding  taken  in 
Belgium. 


Feet. 

Inches. 

Feet. 

Inches. 

Feet. 

Inches. 

Feet. 

Inches. 

1,000 

2,000 

3,000 

4,000 

5,000 

28.88 
27.80 
26.75 
25.75 
24.79 

6,000 

7,000 

8,000 

9,000 

10,000 

23.88 
22.99 
22.11 
21.29 
20.49 

11,000 

12,000 

13,000 

14,000 

15,000 

19.72 
18.98 
18.27 
17.59 
16.93 

16,000 

17,000 

18,000 

19,000 

20,000 

16.30 
15.70 
15.10 
14.54 
13.99 

Table  2. — Shoioing  approximately  the  true  speed  to  the  air  lohen  the 
air-speed  indicator  reads  100  miles  per  hour. 


Feet. 

Miles 

per 

hour. 

Feet. 

Miles 

per 

hour. 

Feet. 

Miles 

per 

hour. 

Feet. 

Miles 

per 

hour. 

1,000 

2,000 

3,000 

4,000 

5,000 

102 
104 
106 
108 
110 

6,000 

7,000 

8,000 

9, 000 

10,000 

112 

114.5 

116.5 

118 

121 

11,000 

12,000 

13,000 

14,000 

15,000 

123.5 

126 

128 

130.5 

133 

16,000 

17,000 

18,000 

19,000 

20,000 

135.5 

138.3 

141 

144 

146 

178 


AIR  SERVICE  HANDBOOK. 


179 


Table  3. — Showing  what  the  graduations  on  the  air-speed  indicator 
represent  at  10,000  feet  and  20,000  feet. 


Feet. 

Graduations. 

30 

40 

50 

60 

70 

80 

90 

100 

110 

120 

130 

10,000 

36.4 
51.4 

48.5 
58.6 

60.5 
73.4 

72.6 

84.8 

97 
117 

109 
132 

121 
146 

133 

145 

157 

20,000 

88.0 

102.5 

The  outside  circle  above  shows  the  graduations  on  the  air-speed 
indicator  and  are  taken  as  correct  at  mean  sea  level.  The  two  inner 
circles  are  the  proper  speeds  through  the  air  at  the  heights  of  10,000 
feet  and  20,000  feet.  Thus,  when  flying  at  10,000  feet,  if  the  indi- 
cator reads  50  miles  per  horn*  the  true  speed  is  about  60. 

T.A.BLE  5. —  Variation  of  velocity  of  the  wind  iviih  height  during  the  day- 
time. 
[Miles  per  hour.] 


Surface  at  Upavon. 

500 
feet. 

1,000 
feet. 

2,000 
feet. 

3,000 
feet. 

4,000 
feet. 

5,000 
feet. 

5.. 

..}           7 

8 

8 

8 

10 

13 

10.. 

15 

18 

18 

18 

19 

20 

15.. 

21 

26 

28 

29 

29 

29 

?n 

. . .  1          28 

34 
43 

37 
47 

40 
49 

40 
50 

40 

25.. 

...[          35 

50 

Table  6. —  Variation  of  direction  of  wind  with  height. 

Wind  veers  with  increasing  height,  i.  e.,  upper  wind  blows  from  a 
point  to  the  right  (or  in  a  clockwise  direction)  of  that  from  which  the 
surface  wind  blows. 

The  amount  of  deviation  from  the  direction  of  the  surface  wind  is 
given  below. 


Sur- 
face. 

500 
feet. 

1,000 
feet. 

2,000 
feet. 

3,000 
feet. 

4,000 
feet. 

5,000 
feet. 

Deviation  to  right,  in 

0 

5 

10 

16 

19 

20 

21 

180 


AIR  SERVICE  HANDBOOK. 

Table  7. — The  Beaufort  wind  scale. 


Beaufort  No. 

Description  of  wind. 

Mean  wind 
force  in 
pounds 

per  square 

foot  at 

standard 

density. 

Equiva- 
lent veloc- 
ity, miles 

per  hour. 

1 

[■Light  breeze 

f          0.01 

J             .08 

1             .28 

.67 

/           1.31 

1           2.3 

/           3.6 

\           5.4 

f            7.7 

\         10.5 

14.0 

1  17.0 

2 

2 

5 

J-Moderate  breeze 

4 

15 

5 

21 

6 

vStrong  wind 

27 

7 

35 

loale  forces 

9 

50 

10 

59 

11 

68 

12 

Hurricane 

175 

1  Above. 


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